Check-preconditions NPM

check-preconditions

A small and simple preconditions library for Node and the browser.

check-preconditions enables easy addition of preconditions to any package, whether ES6, CommonJS, or directly in the browser. With zero dependencies and is <1KB gzipped, it includes type, existence, and equality checks with a convenient BDD-style API. Fully unit tested and ready to go.

Installation

Node

$ npm install check-preconditions

Browser

<script src="lib/check-preconditions.min.js"></script>

Usage

var CheckPreconditions = require("check-preconditions"); // For the minified version: window.CheckPreconditions
var check = CheckPreconditions.check;

var blah = null;
check(blah).is.a.string(); // Throws "Check failed: null was not a String"
check(blah, "blah").is.a.string(); // Throws "Check failed: blah (null) was not a String"
check(blah).is.not.a.string(); // OK!

var result = check(blah).is.null(); // OK! All checks return the value passed in, so result === blah.

A check will throw when its assertion fails and will otherwise do nothing. For instance, check(null).is.a.string() will throw an Error, where check(null).is.null() will not.

is, a, and an can be used to make human readable checks, terminating in a function call defining the check itself.

not negates the check, so check(null).is.not.a.string() will not throw.

An optional name can be given with the object to check and will be included in the Error when thrown. All checks return the original value passed in and typing is preserved for TypeScript consumers.

Named instances

An instance of check can be bound to a name, such as a class or module name. This is useful for tracking precondition failures back to their source in situations where you don't have a stacktrace.

var check = CheckPreconditions.of("Logger");

var blah = null;
check(blah).is.a.string(); // Throws "[Logger] Check failed: null was not a String"
check(blah, "blah").is.a.string(); // Throws "[Logger] Check failed: blah (null) was not a String"
check(blah).is.not.a.string(); // OK!

var result = check(blah).is.null(); // OK! All checks return the value passed in, so result === blah.