2.4.3 • Published 6 years ago

gem v2.4.3

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6 years ago

"Modular, composable web components. Modular, composable styles."


Gem.js is a view library - a set of extensible web components for building visual user interfaces and styling web applications in pure-javascript.

The power of functions and variables is unparalleled, and yet languages like HTML and CSS, which don't have any ability to compose structures together, are still the primary ways people build web applications. Framework after framework has come along to try and solve the problem of making web pages dynamic. Angular, Backbone, Ember, jQuery, mooTools, Dojo, YUI, etc have all tried to be everything to everyone. But what they fail at providing is simplicity and modularity. They end up with bloated libraries filled with features you're not using.

Gem.js is here to change that. Finally, modern application development for the browser!


var Button = require("gem/Button")
var Style = require("gem/Style")
var Block = require("gem/Block")

var list = Block()
;[1,2,3].forEach(function(n) {
    var text = "Hi "+n
    var toggleButton = Button(text) // create a button

    // make it do stuff when you click on it
    toggleButton.on('click', function() {
        if(toggleButton.text !== "RAWRR!!!") {
            toggleButton.text = "RAWRR!!!"
            toggleButton.state.set('color', 'rgb(128, 0, 0)')
        } else {
            toggleButton.text = text
            toggleButton.state.set('color', 'black')

    // add the button to the list

var greet = Button("send", "Send your Greetings") // labels like 'send' can differentiate
                                                  // otherwise identical types of gems

// create styles with style objects ..
list.style = Style({
    border: '1px solid blue', // .. that use familiar css values,
    marginRight: 34,          // .. camelCase css properties and integers interpreted
                              //    as "px" values when appropriate,

    Button: {                 // .. sub-gem styles,
        $$firstChild: {       // .. pseudo-class styles,
            color: 'rgb(0,100,240)',
        $state: function(state) { // .. more sophisticated styling techniques
            return Style({
                color: state.color
    $send: {              // .. style based on an object's label, and ..
        color: 'green'

// append the list of buttons to the document body (so it shows up)

// custom gems (use your favorite javascript class library - here proto is being used)
var NameInput = proto(Gem, function() { // inherit from Gem
    this.name = 'NameInput'
    this.build = function(LabelText) {  // the `build` method initializes the custom Gem
        this.nameField = TextField()
        this.add(Text(LabelText), this.nameField)
        this.nameField.on('change', function() {
            list.children[0].text = "Hi "+this.val

var nameInput = NameInput("Your Name: ")
greet.on('click', function() {
    var text = Text("Whats up, "+nameInput.nameField.val+'?')
    text.style = Style({display:'block'})


If anything in the documentation is unclear, or you want to see more examples, the unit tests give a comprehensive and exhaustive set of examples to look at.

Why use Gem.js?

  • Makes your web application easier to develop with modular reusable structure objects (Gem objects) and Style objects
  • No HTML Needed. With Gem.js, you write in 100% javascript. The only html requirement is a document body. You can still add plain old HTML into your gems using gem.domNode.innerHTML tho if you so choose.
  • No CSS Needed. While Gem.js uses css style properties, it rejects the cascading nature of css, allowing one style to be fully isolated from another. No more wondering which selector in which stylesheet botched your nice clean style.
  • Works with your HTML and CSS. Gems can be added as a child to any standard dom object and they can be styled with standard css stylesheets if you so choose.
  • Works with your current javascript. Gems give you direct access to their domNode so you can use the dom manipulation libraries you're used to.
  • Fully separate style from structure. By using $state, $setup, and $kill javascript in your Style objects, you can include any javascript that is stylistic rather than structural.
  • Import Gem modules with real APIs that anyone can release online. HTML snippets are so 1995.
  • Lowers your risk of Cross-Site scripting. Data in Gem.js doesn't need to be escaped, and so you can cross that off your worry-list.
  • Unlike HTML web components, Gem.js works in modern browsers without polyfills.
  • Also unlike HTML web components, element name collision isn't a problem.
  • Has a small footprint: 16.5KB minified and gzipped in umd format



npm install gem

or download the built package Gem.umd.js from the 'dist' folder in the repository


var Gem = require('gem')  // node.js and webpack

define(['Gem.umd.js'], function(gem) { ... } // amd

<script src="Gem.umd.js"></script>
  Gem; // global 'Gem' module object


All gems inherit from Gem - the basic building-block of the system. Gems are EventEmitters, and emitting events is one of the primary ways gems should communicate.

Gem is abstract and can't be instantiated on its own. See the section ''Custom Gems'' for details on how to create objects that inherit from Gem.

Static properties and methods

Gem.name - The name of the Gem. Used both for naming dom elements for view in browser dev tools and for styling.

Gem.attach(listOfGems) - Appends the passed gems to document.body. IMPORTANT: only attach a gem to the dom via this attach function or a gem's attach/attachBefore method. Without this, styles won't be rendered. \ Gem.attach(domNode, listOfGems) - Appends the passed gems to the passed domNode. \ Gem.attachBefore(domNode, listOfGems) - Appends the passed gems before the passed domNode as a sibling.

Gem.detach(listOfGems) - Removes the passed gems from their respective dom parents.

Gem.createBody(callback) - Dynamically creates the body tag. Calls callback when done.

Gem.dev - (Default: false) - When set to true, certain debugging feature are enabled.

Instance properties and methods

gem.parent - The Gem's parent (which will also be a Gem) \ gem.children - An array of the Gem's children (which will all be Gems themselves). \ gem.domNode - The Gem's standard dom node object. \ gem.label - A string used for styling. Should be set once when the object is instantiated, and cannot change. See the section on Style objects for details about how this is used. \ gem.excludeDomEvents - A set of dom events to exclude from automatic registration. Will have the structure {eventName1:1, eventName2:1, ...}. See the documentation for on for more details. \ gem.state - An observer object that can be listened on for changes. Can be used for any purpose, but is intended for being used to create dynamically changing styles. See the section on Style objects for an example.

gem.add(gem, gem, ...) - Appends gems as children to the calling gem. This causes the domNodes of the passed gems to be appended to the calling gem's dom node. \ gem.add(listOfGems) - Same as above, but listOfGems is an array of Gem objects.

gem.addAt(index, gem, gem, ...) - Adds gems as children to the calling gem at a particular index. \ gem.addAt(index, listOfGems) - Same as above, but listOfGems is an array of Gem objects.

gem.addBefore(beforeChild, gem, gem, ...) - Adds gems as children to the calling gem before a particular child. If beforeChild is undefined, this will append the given nodes. \ gem.addBefore(beforeChild, listOfGems) - Same as above, but listOfGems is an array of Gem objects.

gem.remove(gem, gem, ...) - Removes the passed gems as children. \ gem.remove(listOfGems) - Same as above, but listOfGems is an array of Gem objects. \ gem.remove(index, index, ...) - Removes, as children, the gems at the given indexes in the children list. \ gem.remove(listOfIndexes) - Same as above, but listOfIndexes is an array of indexes to remove.

gem.attach(domNode=document.body) - Appends this Gem's domNode to the passed domNode (default document.body). IMPORTANT: only attach a gem to the dom via the attach function or a gem's attach/attachBefore method. Without this, styles won't be rendered. \ gem.attachBefore(domNode=document.body) - Appends this Gem's domNode before the passed domNode (as a sibling). gem.detach() - Removes this Gem's domNode from its dom parent.

gem.attr(attributeName) - Return the value of the attribute named attributeName on the Gem's domNode. \ gem.attr(attributeName, value) - Sets the attribute to the passed value. \ gem.attr(attributeObject) - Sets the attributes in the attributeObject, where attributeObject looks like: {attribute1: value1, attribute2: value2, ...}.

gem.style - Holds the object's Style object. Starts out undefined, and can be set to undefined to remove a Style that has been set. Changing this property triggers style affects in the Gem's children. \ gem.visible - Setting this variable to false hides the gem using "display: none;". Setting this variable to true unhides it. Accessing the variable will return its visibility state. \ gem.focus - Setting this variable to true gives the gem focus on the page. Setting this variable to false blurs it. Accessing the variable returns whether or not the gem is the focused element on the page. \ gem.quiet.focus - Just like gem.focus but won't cause a "focus" or "blur" event.

gem.selectionRange - Returns an array representing the selection range in terms of visible character offsets. E.g. a value of [2,4] means that the current element has 2 visible entities (usually characters) selected within it at offset 2 and 4 from the start. Note that if there are hidden characters like multiple spaces in a row, or newlines, or other non-visible characters (mostly only applies to contenteditable nodes), they are ignored. \ gem.selectionRange = [offsetStart, offsetEnd] - Setting the selectionRange property sets the selection inside the Gem's domNode based on the given offsets.

Example of selectionRange:

var x = Text("You're not my buddy, guy")
x.selectionRange = [0,6] // selects "You're"

Event instance properties and methods

Most of these are inherited from EventEmitter.

All methods and properties from EventEmitter are inherited by Gem. The important ones:

gem.emit(event, data, data2, ...) - Emits an event that triggers handlers setup via the Gem's on methods.

gem.on(event, callback) - Registers a callback that will be called when the passed event is emitted by the Gem. \

  • event - The string event name to listen for. If the passed event is one of the many standard dom events (e.g. 'click', 'mouseover', 'touchstart', etc), the passed handler will be registered as a dom event handler in one of three cases:
    • the gem's excludeDomEvents object is undefined
    • the event is in the gem's excludeDomEvents property
  • callback(data, data2, ...) - the callback gets any arguments passed to emit after the event name.

gem.onCapture(event, callback) - Just like gem.on but listens on the capture phase of native browser events. Note: this doesn't currently listen on events that aren't native browser events.

gem.once(event, callback) - Like on but the callback will only be called the first time the event happens.

gem.off(event, callback) - Removes a callback as an event handler (the callback won't be called for that event again). gem.removeListener(event,callback) - Same as off.

gem.removeAllListeners(event) - Removes all the callbacks for the passed event, except capture handlers. gem.removeAllListeners() - Removes all callbacks except capture handlers.

gem.offCapture(event, callback) - Removes a capture handler.


The ifon and related methods are useful primarily for performance reasons. They allow registering event listeners only when they're needed, so that the browser doesn't get overloaded with event handlers. Its recommended that ifon is used whenever possible.

An example:

var text = Text("CLICK ME")
var parent = Block(text)

var handler;
parent.ifon('someoneClickedTheThing', function() {
    text.on('click', handler = function() {
        parent.emit('someoneClickedTheThing', "I can't believe it")
parent.ifoff('someoneClickedTheThing', function() {
    text.off('click', handler)

gem.ifon(event, callback) - Registers a callback that will be called when a handler is registered for event if it had no handler registered previously. If there is already a listener attached to that event, callback is called immediately. \ gem.ifon(callback) - Registers a callback that will be called when the first handler for any event is registered.

    • callback(event) - The callback gets the newly registered event type as its argument.

gem.ifoff(event, callback) - Registers a callback that will be called when the last handler for event is unregistered. \ gem.ifoff(callback) - Registers a callback that will be called when the last handler for any event is unregistered.

  • callback(event) - The callback gets the unregistered event type as its argument.

gem.removeIfon() - Removes all ifon handlers. \ gem.removeIfon(event) - Removes all ifon handlers for the passed event. \ gem.removeIfon(callback) - Removes callback as an "all" ifon handler (a callback passed to ifon without an event). \ gem.removeIfon(event, callback) - Removes callback as an ifon handler for the passed event.

gem.removeIfoff() - Removes all ifoff handlers. \ gem.removeIfoff(event) - Removes all ifoff handlers for the passed event. \ gem.removeIfoff(callback) - Removes callback as an "all" ifoff handler (a callback passed to ifoff without an event). \ gem.removeIfoff(event, callback) - Removes callback as an ifoff handler for the passed event.


The proxy method uses ifon and ifoff to minmize the number of event listeners that need to be attached in the system.

gem.proxy(emitter, options) - Proxies event registration to emitter. \

  • emitter - The emitter (usually a Gem) to proxy handler binding to \
  • options - An object that defines what events are proxied. If undefined, all events are proxied. The object can have one of the following properties:
    • only - An array of events to proxy.
    • except - An array of events to not proxy. All other events are proxied.

Example of proxy:

var A = Text()
var B = Text()

A.on("click", function(x) {
    console.log("hey hey heyyy! "+x)

B.emit("click", "Ughh..") // console prints "hey hey heyyy! Ughh.."

Instance events

"attach" - Emitted when the gem is attached to the document. \ "detach" - Emitted when the gem is detached from the document. \ "newParent" - Emitted when a Gem gets a new parent. Note: this event is used by Style objects, so don't prevent these events. \ "parentRemoved" - Emitted when a Gem is detached from its parent. Note: this event is used by Style objects, so don't prevent these events. \

Dom Events

Gem object will emit any standard dom event ("click", "mousedown", "keypress", etc) when listened on. Note that a Gem doesn't add an event listener to the dom node until someone listens on that event on the gem. This minimizes the number of event listeners that are registered on the page. To see the list of dom events this applies to (supposed to be all of them), see the top of src/node_modules/Gem.js

Custom Gems

Gem.js is all about custom components. That's the point: your application should be built as a composition of custom gems on top of custom gems so that, instead of a million divs, you have semantically appropriate javascript web components.

In this documentation, we're going to be using the class library proto. The descriptions here apply to both inheriting from Gem and inheriting from any of the standard gems. There are a couple special properties to create when making a custom Gem:

  • name - The name is a required property, should be named whatever your class is named, and should be a somewhat unique name in your system (tho it isn't required to be unique).
  • build() - The "sub-constructor". The constructor calls this method, passing all arguments, to the build method. The return value of build is ignored.
  • defaultStyle - If set to a Style object, the style object will be the gem's default style. Unlike explicitly set Styles and inherited Styles, css properties in defaultStyle do cascade line-by-line. Also, if a gem inherits from another Gem class that also has a defaultStyle, the default styles mix together with the child Gem class style properties overriding the parent Gem class's default properties. So in the below example, if gem is given a style that defines color: green, it's fontWeight will still be 'bold'.

For example:

var CustomGem = proto(Gem, function() {
    this.name = "CustomGem"
    this.defaultStyle = Style({
        color: 'red',
        fontWeight: 'bold'

    this.build = function(x) {
        this.x = x

var gem = CustomGem(5) // gem.x is 5

Releasing custom gems as separate modules

If you'd like to release a custom Gem or set of Gem objects, there are a couple of important things to remember to do:

  • If you're releasing on npm, do not add Gem.js as a normal "dependency". Instead, it should be added as a "peerDependency" or perhaps a "devDependency". It shouldn't be a normal "dependency" because otherwise bundlers may bundle multiple copies of Gem.js when using your custom gem module (even though bundlers like webpack dedupe files, if the versions of webpack being used are slightly different, they would still package together both versions of Gem.js)
  • If you're releasing a module distribution intended to be loaded in a <script> tag, do not bundle Gem.js in your distribution bundle. It should assume the gems global variable (e.g. gems.Gem) is available.

Inheriting from Gems with a class library other than proto

If you're building Gems with something other than proto (or are using a version of proto older than 1.0.17), note that Gem.js relies on the following properties:

  • gem.constructor - must point to the Gem prototype class (in the proto example, the object returned by the call to proto). This is a standard property that all good class libraries should set.
  • gem.constructor.parent - must point either to the parent of the gem's constructor, or undefined if there is no parent. Note that while proto sets this automatically, it is not a standard property and if you're using a different library from proto, you must set this manually.
  • gem.constructor.name - the constructors must have the same name property that instances can access. Note that while proto sets this appropriately, most class libraries probably don't and it isn't simple to manually set. See here for details.

Also, make sure that Gem's constructor is called on new instances that inherit from Gem.

Inheriting from Gems without a class library

Properly subclassing a prototype in javascript isn't the simplest thing to do, but if you want to do it, here's how:

var CustomGem = function() {
    Gem.init.call(this) // Gem's constructor must be called
CustomGem.parent = Gem // needed for correct Style rendering

var Intermediate = function(){}; Intermediate.prototype = Gem.prototype
CustomGem.prototype = new Intermediate()
CustomGem.prototype.name = 'CustomGem'       // the name is a required property
CustomGem.prototype.constructor = CustomGem  // required for correct Style rendering,
                                             // and is a standard javascript convention
CustomGem.prototype.build = function(constructorArgument1, constructorArgument2, ...) {
    // .. custom constructor code
CustomGem.prototype.customMethod = function() {
    // ...

Standard Gems

The built-in standard gems all inherit from Gem and so have all the methods and properties in the above documentation. For each build-in gem, its name property will be the same as the name the documentation uses for it. For example Button will have the name "Button".

To use these built in gems, access them via either require("gem/<GemName>") or Gem.<GemName>. For example:

var Table = require("gem/Table") // webpack or browserify
// or
var Table = Gem.Table // if loading the umd bundle in a <script> tag


There are some conventions that can help you learn to use standard Gems, and help make custom Gems you build more easily understood. These conventional properties, constructor parameters, and behavior are encouraged to be used in custom Gems built by you, especially if you're planning on open-sourcing them.

Every standard Gem has an optional first parameter label. This makes it easy and non-intrusive to label parts of your custom Gems for easy styling.

In as many cases as possible, Gems will use properties defined with getters and setters rather than using methods. There are a few standard properties that some gems have:

  • text - Gets and sets some static visual text that a Gem has. Button, Text, and Select.Option have this property.
  • selected - Gets and sets the selected-state of the Gem. CheckBox, Select.Option, and Radio.Button have this property.
  • val - Gets and sets some value that a gem has. This will never be the same as either text or selected. Radio, Select, TextArea, and TextField all have this property.
  • quiet - Any gem that has a selected or val property will have a corresponding quiet.selected or quiet.val property that works in the same way, except that it doesn't emit any change events.

This is a standard event that many gems can emit:

  • change - Emitted when an important value of a gem changes. This will always be either the gem's val property or its selected property (but never both). Change events won't have any information passed with them - you can access the object itself if you need data from it.

Some gems have sub-gems specifically related to them. For example, Select has Option gems, and Table has Row and Header gems, and Row and Header have Cell gems respectively.

  • There will also be a property with the name of the sub-gem, but lower-case and plural, that contains either a map or a list of the sub-objects. For example, Select has an options map.
  • For these types of gems, there will be a method on the main gem (examples of main gems: Select or Table) to create a new sub-gem (e.g. Option or Row), append it to the calling gem, and returns that sub-gem. The method will be named the same as the sub-gem but in lower-case (e.g. selectGem.option(...) will return an Option gem).


Your standard html <button>.

Button(text) - Returns a new button that has the passed text. \ Button(label, text)

button.text - Sets or gets the button's text.


Your standard html <canvas>.

Canvas(height, width) - Returns a new Canvas object that has the passed dimensions. \ Canvas(label, height, width)

canvas.height - Sets and gets the canvas's height. \ canvas.width - Sets and gets the canvas's width.

canvas.context(type, attributes) - Returns a standard canvas context. The type and attribute parameters and return value are the same as the html-standard getContext.

canvas.toImg - Returns a data-url representing the image currently drawn on the canvas. \ canvas.toDataURL - Same as canvas.toImg.


Your standard html <input type="checkbox">.

CheckBox() - Returns a new unchecked CheckBox. \ CheckBox(label)

checkbox.selected - Sets and gets the checkbox's selected state (true for selected, false for unselected). \ checkbox.quiet.selected - Same as selected but emits no change event.


A <div> that contains other gems.

Block(gem, gem, ...) - Returns a new container with all the passed gems as children (in the passed order). \ Block(listOfGems) - Same as above except listOfGems is an array of gems. \ Block(label, gem, gem, ...) \ Block(label, listOfGems)


Your standard html <img>.

Image() - Returns a new empty image. \ Image(imageSource) - Returns a new image with the passed imageSource. \ Image(label, imageSource)

image.src - Gets or changes the image's source.


An <ol> or <ul> element.

List() - Returns a new empty list. \ List(ordered) - Returns a new empty list. Is an ordered-list if ordered is true, and an unorderd-list otherwise. \ List(listInit) - Returns a new populated list. listInit is an array containing either Gem objects or strings to add as list items. \ List(ordered, listInit) \ List(label) \ List(label, ordered) \ List(label, listInit) \ List(label, ordered, listInit)

list.item(contents) - Creates a new ListItem with the passed contents and appends it to the list, which can either be a Gem or a string. \ list.item(label, contents)

List.Item - The ListItem class.

List.Item(contents) - same as list.item above, except doesn't append the item to any list. \ List.Item(label, contents)

Radio - (Not a Gem)

A set of radio buttons. Radio itself is not a Gem, but rather contains a set of related RadioButtons (which are Gem objects).

Radio() - Returns a new Radio object where a button is not required to be set (same as Radio(false)). Radio(required) - Returns a new Radio object. If required is true, a radio button will always be selected (and buttons cannot be deselected), otherwise radio-buttons can be deselected, and no radio button is selected by default.

radio.button(value) - Creates a new RadioButton with the passed string value that is a member of the Radio object. radio.button(label, value)

radio.selectedOption - Returns the RadioButton object that is selected. \ radio.val - Gets the value of the RadioButton that's selected, or selects the RadioButton that has the set value (e.g. radio.val = 'elvis' would select the radio button with the value "elvis") \ radio.quiet.val - Same as val but emits no change event.

radio.remove(radioButton, radioButton, ...) - Removes the passed radio buttons from the Radio object's set. Note that this will not remove the buttons from the page - that must be done separately for whatever Gem contains the RadioButtons. radio.remove(arrayOfRadioButtons) - Same as above, except the argument is an array of the RadioButtons to remove. radio.remove(value, value, ...) - Removes the radio buttons that have the passed values from the Radio object's set. radio.remove(arrayOfValues) - Same as above, except the argument is an array of the values who's radio buttons should be removed.

Radio.Button - The RadioButton class.

radioButton.val - Gets or sets the value of the radio button. \ radioButton.selected - Gets whether the radio button is selected or not. If set to true, selects the button. If set to false, deselects it. \ radio.quiet.val - Same as val but emits no change event. radioButton.selectNext() - Sets the next radio button in the Radio object's set. \ radioButton.selectPrevious() - Sets the previous radio button in the Radio object's set.


Your standard <select> element.

Select() - Returns a new empty selection list. \ Select(selections) - Returns a new populated selection list. \ Select(label, selections)

  • selections - An object with the structure {optionValue: optionText, ...}

select.option(value, text) - Creates a new Option with the passed value and text, and appends it to the list. \ select.option(label, value, text)

select.options - A map where each value is a Select.Option gem the Select instance contains, and each key is the value of that option. \ select.val - Gets the value of the selected Option, or selects the Option with the set value (e.g. select.val = 'moo' selects the Option with the value 'moo'). \ select.quiet.val - Same as val but emits no change event.

Select.Option - The Option class.

Select.Option(value, text) - same as select.option above, except doesn't append the Option to any list. \ Select.Option(label, contents)

option.selected - Gets or sets the selected state of the Option. \ option.text - Gets or sets the display text of the Option. \ option.val - Gets or sets the string value of the Option. \ select.quiet.selected - Same as selected but emits no change event. \ select.quiet.val - Same as val but emits no change event.


An <svg> element.

Svg(svgXml) - Returns a new Svg image using the passed svgXml. The svgXml must include the <svg> tag at the top-level. \ Svg(label, svgXml)


Your standard <table> element.

Table() - Returns a new empty table. \ Table(tableInit) - Returns a new populated table. \ Table(label,tableInit)

  • tableInit - A list where each element in the list represents a row. Each element itself should be a list where each element is a Gem or string to put in a table Cell. E.g. Table([['a','b','c'],[Text('x'),Text('y')]]) is a table with two rows and three columns, where there are only two cells in the second row.

table.row(rowInit) - Creates a new table TableRow (<tr>), and appends it to the table. \ table.row(label, rowInit) \ table.header(rowInit) - Creates a new table TableHeader (<th>), and appends it to the table. \ table.header(label, rowInit)

  • rowInit - A list where each element is a Gem or string to put in a table TableCell. E.g. table.row(['a','b','c']) is a row with three cells.

Table.Row - The TableRow class. \ Table.Header - The TableHeader class. \ Table.TableCell - The TableCell class.

Table.Row(rowInit) - same as table.row above, except doesn't append the TableRow to any table. \ Table.Row(label, rowInit) \ Table.Header(rowInit) - same as table.header above, except doesn't append the TableHeader to any table. \ Table.Header(label, rowInit)

row.cell(contents) - Creates a new table TableCell (<td>) and appends it to the TableRow. \ row.cell(label, contents) \ header.cell(contents) - Creates a new table TableCell (<td>) and appends it to the TableHeader. \ header.cell(label, contents)

  • contents - Either a string (text content) or any value you could pass into gem.add (a Gem, a list of Gems, etc).

Table.Cell(contents) - Same as row.cell, but doesn't append the cell to any row. \ Table.Cell(label, contents)

cell.colspan(columns) - Sets the column-span (colspan attribute) of the cell.


A <div> with text in it.

Text() - Returns an empty Text object. \ Text(text) - Returns a Text object populated with the passed string text. \ Text(label, text)

text.text - Sets or gets the object's text.


A multi-line text input field. Your standard <textarea> element.

TextArea() - Returns an empty TextArea. \ TextArea(label)

textArea.val - Gets or sets the testArea's value (the text inside the text box). \ textArea.quiet.val - Same as val but emits no change event.


A one-line text input field. Your standard <input type='text'> element.

TextField() - Returns an empty TextField. \ TextField(password) - Returns an empty TextField with the password attribute, meaning any text inside the box will be displayed so that only the number of characters can be seen, and not the characters themselves. \ TextField(label, password)

textField.val - Gets or sets the textField's value (the text inside the text box). \ textArea.quiet.val - Same as val but emits no change event.

Style objects


If you're going to build a web application, why not do it with Style?

While a Gem is pretty analogous to its HTML node, Style objects in Gem.js are quite different from normal CSS.

In Gem.js, individual css style properties do not cascade. Instead, whole Style objects cascade. This may not seem like much of a difference, but it makes all the difference. For example:

var parentBlock = Block([

c.style = Style({
    Text: {           // 1
        color: 'blue',
        fontWeight: 'bold'
    Block: {
        Text: {       // 2
            color: 'red'

In the above example, "a" will be bold and blue, and "b" and "c" will be red. But "b" and "c" won't be bold - that property does not cascade. The Text styling inside Block is isolated from all previous stylings of Text, which means you don't have to worry about the styles someone used for elements further up the dom tree. I mentioned, tho, that whole Style objects do cascade, and that is why "d" will also be blue and bold even though it isn't a direct child of parentBlock.

Another difference is that Gem.js doesn't have selectors that can style any element on the page. Traditional CSS stylesheets are developed by selecting a group of elements from the entire page (via ids, classes, attributes, pseudoclasses representing element state, etc) and appending styles to them. These styles may overwrite styles written earlier, and they themselves may be overwritten. In Gem.js, Style objects can only be attached in a strict hierarchical setting, where only a specific section of the dom can be affected. In the above example, the Text style marked 2 doesn't affect anything outside that inner Block. For example, even though the text "d" is a Text object inside a Block object, it is not colored red. That's because styles in Gem.js are not like the selectors you're used to from css. They are strictly hierarchical - they only affect descendant Gem objects' (children, grandchildren, etc) from the point in the dom they match.

The combination of the fact that Gem.js Style objects only cascade as a whole object and that styles are defined hierarchically makes styles modular and provides isolation from other styles on the page, so that it becomes much easier to understand and manage styling for a page or entire application.

Style constructor

Style(styleDefinition) - Creates a Style object.

  • styleDefinition is an object where key-value pairs can be either style properties or selectors:
    • style properties:
      • <cssPropertyName>: the value is a valid css value for that style property.
      • $setup: the value is a function to be run on a gem when the style is applied to it
      • $kill: the value is a function to be run on a gem when a style is removed from it
      • $state: the value is a function to be run when gem.state changes (ie when its change event fires).
    • selectors - the value for each of these should be a Style object or a nested styleDefinition object
      • <GemName>
      • $<label>
      • $$<pseudoclass>
    • $inherit


This type of key-value pair is simple - just your basic css style. Example:

    color: 'rgb(100, 200, 50)',
    marginRight: 3

The above style would give a color and margin-right to whatever Gem is set with that style (gem.style = styleObject). Note that camelCase names can be used, and numbers are automatically appended with "px" if appropriate for the property (just like with jquery's css method).

$setup and $kill

$setup(gem, style) - A function that is run when the style is applied to a gem. The passed style parameter is the style containing the $setup function.

$kill(gem, setupObject) - Run when the style is removed from a gem. Also gets the return value of the $setup function as its second argument.

For example:

var S = Style({
    $setup: function(gem) {
        gem.text = "I got zee style"
        return 20
    $kill: function(gem,setupValue) {
        gem.text = "I'm "+setupValue+"% less cool"
var t = Text("x")
t.style = S
t.text === "I got zee style"
t.style = undefined
t.text === "I'm 20% less cool"


$state(state) is a function that is run when the gem's state observer property emits a change event (which happens when its changed with its methods set, push, splice, or append). The state function is passed the value of gem.state.subject as its parameter. The return value of the function should be a Style object to mix with the object's current style.

Note that if you create styles in $state functions, remember that you will create a new style every time the state changes. This may be a problem with some applications that have a lot of state changes, or particularly rapid state changes (its unclear at what point this could cause problems).


var c = Text("hi")

var colorStyles = {
    yellow: Style({backgroundColor: 'yellow'}),
    red: Style({backgroundColor: 'red'}),
    green: Style({backgroundColor: 'green'})
c.style = Style({
    color: 'blue',
    $state: function(state) {
        if(state.success) {
            if(state.late) {
                return colorStyles.yellow
            } else {
                return colorStyles.green
        } else {
            return colorStyles.red

c.state.set("success", true) // container's background turns green
c.state.set("late", true)    // container's background turns yellow

In the above example, the container goes through all 3 backgroundColor colors as its state changes. In all states, the text color will be blue tho, since the $state style mixes with the primary style properties.


This sets a style for the Gem with the given name. Only gems within the gem who's style this is are affected, gems that do not descend from the styled gem, and even the styled gem itself is not affected. For example:

var stylish = Style({
    Text: {
        color: 'red'

var container = Block([Text('a')])
var text = Text("b")

container.style = stylish
text.style = stylish

In the above example, only "a" is styled red. The Text "b" remains the default, black.

You can also give Gems a Style object, which is the same as the above form, except that the object-immediate is passed into the Style constructor:

var textStyle = Style({
    color: 'red'

var stylish = Style({
    Text: textStyle

Keep in mind that Gem styles are inherited by their parent in-full (not line-by-line like in css). So any Gem that isn't given its own explicit style inherits from its ancestors (if its ancestors have a style for that Gem). For example,

var a,b,c,d,e, dBlock
var tree = Block([
    a = Text('a'),
        b = Text('b'),
        c = Text('c'),
        dBlock = Block([
            d = Text('d'),
                e = Text('e')

tree.style = Style({
    Text: {color: 'green'}
c.style = Style({
    fontWeight: 'bold'
dBlock.style = Style({
    Text: {textDecoration: 'underline'}

I hope you don't mind I assigned variables inside the Gem structure - don't worry, the structure would be the same if the variables weren't assigned, but assigning the variables allows me to manipulate them after the structure is defined.

In any case, in the above example, the tree object is given a style where Text gems are green. However, dBlock is given an alternate Text style. How does that affect the resulting style? Well, "a" and "b" are colored green, but "d" and "e" are not green. Instead, "d" and "e" are the default color, black, and are underlined. This is because "d" and "e" inherit the Text style from dBlock which overrides the Text style given to tree. And what about "c" you ask? Since "c" is given its own explicit style, that style also overrides any inheriting style. So "c" is bold and the default black.


Gem objects can be given a label property, which can be used to identify Gem objects of the same type that have different purposes within their parent. This is essentially analogous to giving divs class names, and then styling using that class, except that it works in gem.js's hierarchical way of course. Note, tho, that a Gem can have only one label. For example:

var container = Block([
    Text("header", "Your Receipt, Sir: "),
    Text("receipt", "1 bagel - $5, 3 buffulo - $45, 3 sticks dynamite - $100, 45 tons of wheat - $200")
    Text("Thank you for using butler-copter!")

container.style = Style({
    Text: {
        fontWeight: 'bold',
        color: 'blue'
    $header: {color: 'gray'},
    $receipt: {color: 'green'}

In the above example, the text "Your Receipt, Sir: " will be gray, the receipt text will be green, and the thank-you text will be blue. Only the thank-you text will be bold.

Note that $label styles override gem styles, even if the gem style is "closer" to the gem. For example, in the following example, the text "hi" will be gray, not blue.

var container = Block([

container.style = Style({
    $someLabel: {color: 'gray'},
    Block: {
        Text: {color: 'blue'}


Pseudoclasses filter out which Gem styles are given to. Also, the styles mix with the styles defined in the style-definition containing it. For example:

var x = Text("hi")
x.style = Style({
    fontWeight: 'bold',
    $$hover: {color: 'red'}

The above code changes the text from bold-black to bold-red when you hover over it, and back to black again when you move your mouse off the text.

Pseudoclasses also mix with each other when applicable. For example:

var container = Block([Text("a"), Text("b")])
container.style = Style({
    Text: {
        $$lastChild: 'bold',
        $$hover: {color: 'red'}

In the above code, "a" will start black and change to red when you hover over it. "b" will start black and bold, but will change to red and bold when you hover over it.

To set a style only when multiple pseudoclasses apply, simply nest them. For example:

var text = Text("a")
text.style = Style({
    $$lastChild: {
        $$hover: {color: 'red'}

In the above code, "a" will turn red if it is both the last child of its parent and is hovered over.

Also, pseudoclasses may take parameters, which are passed in with parens like a javascript function. For example:

var c = Block([Text("a"),Text("b"),Text("c"),Text("d"),Text("e")])
c.style = Style({
    Text: {
        '$$nthChild(1+2n)': {color: 'red'},

In the above code, "a", "c", and "e" are red, while "b" and "d" are black (the default).


The $inherit property, if set to true, indicates that the style inherits properties from the style "above" it - the style that would be used if the current style wasn't there. For example in the following, the text inside the container gets a green background, but also a red text color:

    Text: {color: 'red'},
    Block: {
        Text: {
            $inherit: true,
            backgroundColor: 'green'

In the following, a Text gem with the label 'boom' would have both a green background and red text, whereas a Text gem without that label, would not have the background color:

    Text: {color: 'red'},
    $boom: {
        $inherit: true,
        backgroundColor: 'green'

Note that $inherit is ignored the top-level of psuedoclass styles, $state styles, and defaultStyles since they always inherit.

Combining them together

Style objects can be as simple as a few standard css properties, or can take the place of a whole css stylesheet and more. Here's an example of a more complex style:

var topBarHeight = 100

    marginTop: topBarHeight,

    Button: {
        $setup: function(gem) {
            var handler;
            gem.on('moo', handler=function() {
                console.log("He won't stop mooing!")
            return {handler:handler}
        $kill: function(gem, setupState) {

    $closeButton: {
        //future: $inherit: true, // inherits the Button style (if the gem is a Button)
        position: 'absolute',
        right: 3, top: 3,
        width: 'calc(50% - 2px)'

    Table: {
        TableHeader: {
            TableCell: {
                borderBottom:'1px solid #000'
        TableRow: {
                TableCell: {
                    borderTop: '1px solid #000'
            TableCell: {
                '$$nthChild(1)': {
                    borderLeft: "1px solid #000"
                $$lastChild: {
                    borderRight: "1px solid #000"
            $$lastChild: {
                TableCell: {
                    borderBottom: "1px solid #000",
                    '$$nthChild(1)': {
                        borderLeft: "1px solid #000",
                        borderRight: "1px solid #000",

    TopBar: {
        marginTop: -topBarHeight+2,
        position: 'absolute',
        right: 0,

        Search: {
            position: 'relative',
            top: -3,

                 height: 23

        DropTab: {
            position: 'static',

            $wrapper: {
                position: 'static'

            $menu: {
                position: 'absolute',
                border: border,
                marginTop: 1,
                backgroundColor: 'white',
                cursor: 'pointer',

                Text: {
                    color: 'bonkers'
            $button: {
                border: border,
                fontSize: 24,
                fontWeight: 'bold'

        $userDropTab: {
            //future: $inherit: true, inherits from DropTab above
            cursor: 'pointer'


styleObject.mix(styleObject2) styleObject.mix(styleDefinition) styleObject.mix(styleObject2, mixInherit) styleObject.mix(styleDefinition, mixInherit)

Returns a new style that combines together calling style object and the passed style object or style definition. The properties from the passed style object override the ones from the calling object.


var newStyle = oldStyle.mix({
   color: 'red'
var newStyle2 = oldStyle.mix(anotherStyle)


Returns a new style that is a copy of the calling style object but has a new className. This was written mainly for internal use.


Style.addPseudoClass(name, options) - Creates a new pseudoclass that can be used in Style objects. This can be used to create all-new psuedoclasses no one's ever thought of before!

  • name - The name of the new pseudoclass
  • options - An object with the members:
    • check(gem, processedParameter) - A function that returns true if the pseudoclass applies to passed gem
    • setup(gem, startCallback, endCallback, processedParameter) - A function that should call startCallback() when the pseudoclass starts applying, and endCallback() when it stops applying. Can return a state object that will be passed to the kill function.
      • processedParameter - The parameter passed to the pseudoclass (e.g. in ":not(:first-child)", ":first-child" is the parameter). If a processParameter function is given, this will be the return value of that function.
    • kill(gem, state) - A function that cleans up any event listeners or anything else set up in the setup function.
    • emulated - (Optional - default false) Set to true if this implements a native pseudoclass. If true, Gems will attempt optimize the pseudoclass using native css if possible.
    • processParameter(parameter) - (Optional) Takes the pseudoclass parameter and returns some object representing it that will be used by the setup and check functions.
    • parameterTransform(parameter) - (Optional) Returns a modified version of the passed parameter that will be used in place of the given parameter in native-rendered psuedoclass styles. This is useful in cases where native pseudoclass parameter parsing is unnecessarily strict (eg. nth-child parameters)


styleObject.toString() - Outputs a serialized representation of the style as a string. Note that $state, $setup, and $kill functions are serialized by using their function's toString method.


Style.fromString(styleString, context) - Converts styleString into a Style object.

  • styleString - Should have originally been output from styleObject.toString()
  • context - An object where each key is a variable name that will be available in any $state, $setup, or $kill functions, and each value is the value that variable will have. Note that the variable Style is already defined for you if you don't specify it in the context.


styleObject.toObject() - Outputs a regular javascript object representing the Style object. If passed back into the Style constructor, will render the same style.

Standard Pseudoclasses

Any pseudoclass that exists in standard css can be used by gem.js, even if it isn't build-in with a js-rendered emulation. The catch is that their use is limited. The following things aren't supported for these:

  • Using a $setup, $kill, or $state functions within the pseudoclass style definition
  • Using non-standard pseudoclasses within the pseudoclass style definition

Built-in JS Rendered Pseudoclasses

  • hover - The usual :hover pseudoclass
  • checked - The usual :checked pseudoclass
  • required - The usual :required pseudoclass
  • lastChild - The usual :last-child pseudoclass
  • nthChild(a+bn) - The usual :nth-child pseudoclass (with more tolerant parameter parsing)

Note that, while the list of built-in pseudoclasses is currently short, all standard pseudoclasses can be defined except the ":visited" pseudoclass, because the necessary information is not available via javascript (a browser "security" policy).

Default style

Gem.js unifies the default styles of dom nodes - all objects that inherit directly from Gem have the same default styling unless they define their defaultStyle property.

The base default is mostly the same as css's base default. The two defaults that are different for Gem objects:

  • display - "inline-block"
  • position - "relative"
  • box-sizing - "border-box" (excepting <img> nodes, which are still defaulted to content-box because pixel perfect image sizing is often important)

Also, while most css styles are not inherited from a Gem's parent, the following are inherited:

  • color
  • cursor
  • fontFamily
  • fontSize
  • fontStyle
  • fontVariant
  • fontWeight
  • visibility

And while gem.js generally rejects css's use of cascading, there is some similar cascading going on, but much more simplified and localized. Defining this behavior in terms of "cascading order" (where later additions override earlier), the order is: 1. The base default style (described above in this section). 2. The defaultStyle property of the gem instance's furthest ancestor class 3. ... (more defaultStyles in the Gem's ancestry) 4. The defaultStyle property of the Gem's parent class 5. The defaultStyle property of the Gem itself 6. The Gem instance's current style (either an inherited style or its style property) 7. The state style returned for that gem by its current style 8. Any pseudoclass styles that apply on the gem

Tips and Recommendations

Don't Create a node until you need it

Often in traditional web development, all the HTML will be rendered and any javascript for them initialized on page load, and the elements that shouldn't be shown to the user are simply hidden, and then shown when needed. Using gem.js, its recommended that you create and remove nodes as needed, rather than showing and hiding (using visible). The only reason to use the show/hide technique is if the element in question is particularly expensive to generate.

Other Gem Modules

Here's a list of other available Gem modules:

  • gemtk - A toolkit of gems for common components.


  • Gem.label is not dynamic (can't be changed) because it is intended to be used to identify a particular Gem when multiple Gems of the same type are used alongside eachother. If you're looking for a way to change styles dynamically, use Gem.state.
  • Gems are styled based on their name rather than their object identity (which would be possible with an array style definition like Text,{backgroundColor:'...'}) because otherwise all Gems would have to be exposed at the top level. Not only does this go against modularity, but creators of 3rd party modules would inevitably fail to expose all their Gems, which would make styling impossible. With names, you don't have to be able to reach the object, you just have to know its name.
  • don't use npm shrinkwrap for browser modules like this, because otherwise multiple minor versions of the same package might get into a browser bundle and make page loading slower

What Gem.js is not

Gem.js is not:

  • a compatibility layer. Gem.js uses modern browser features and is built to rely on polyfills to add older browser support. If you're looking for a compatibility system, try modernizr.
  • a path router. If you're looking for a path routing module, try grapetree.
  • a class system. If you're looking for a system for creating classes and prototypes, try proto.
  • a module system, script bundler, or loader. If you're looking for an excellent module system and bundler, look no further than webpack.
  • a templating system. In gem.js, you use functions and objects to compose together a page, rather than templates.
  • an animation library. If you're looking for animations, try Émile.
  • a framework. A framework is a system that calls your code. A module is a set of functions and classes that your code can call. Gem.js is the latter. Gem.js can work well right alongside traditionally written html and css, and you can choose to wrap dom constructs in a Gem only if you want to.
  • super heroic. It does one thing well: web components. It embraces the single-responsibility principle and is entirely stand-alone.


  • Billy Tetrud
  • Robbert Wijtman - designed and helped implement the prototype version of gem.js


  • Improve error messages for styles
    • Show the selector of the Style that went wrong in the error message
    • Show the name of the gem being styled/restyled to a style that caused the error
    • Even better would be to be able to some context, maybe show the path like "Gem.$inner.LayoutEditor.$wrapper.$$hover" so you can trace where exactly in your style objects things are going wrong
  • consider adding vertical-align:top to the default css, since it solves weird issues with inline-blocks causing extra bottom-margin: http://stackoverflow.com/questions/20310690/overflowhidden-on-inline-block-adds-height-to-parent
  • Emulate the :not psuedoclass

  • Figure out how to support animations, @keyframes and that kind of thing

  • support css animations

  • Maybe if a gem has an explicit style set, it ignore's any styling from its parent (ie the componentStyleMap)

    • Similarly, maybe if a gem has an explicit style set, it shouldn't be able to inherit from anything
  • user-defined style properties
    • parameterized and combinable/overridable
  • Finish MultiSelect (currently may not fire certain events with certain ways of selecting things with the mouse)
  • Make all controls usable via the keybaord

    • eg. checkboxes should be toggled if you press enter while they're focused on
  • Consider making Style objects dynamically changable, and also inheritable/extendable so that you can extend the style object of a Gem instead of having to extend the object passed to a Style prototype

  • in separate module (a Gems utility kit or something):

    • RadioSet (a set of labeled radio buttons)
    • TextEditor

Optimization ideas:

  • When possible, use a WeakMap to cache Style mixes, componentStyleMap conjunctions, etc so that extraneous style don't cause memory leaks
  • When dom nodes are added, either add them inside a setTimeout, or render their styles in a setTimeout (first way's probably best so you have less change of seeing unstyled gems)
    • This should improve UI responsiveness, but not execution time
  • Purge style cache entries after a certain age (for non-weak maps)
  • Use document fragments when adding multiple dom nodes
  • Mix together defaultStyles statically (once per gem type) since they're not dynamic


  • 2.4.3 - Fixing bug in nth child rendering that caused 2+n and n+2 to parse i

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