0.1.1 • Published 1 year ago

circumspect v0.1.1

Weekly downloads
Last release
1 year ago


circumspect is a set of functions to make your TypeScript/JavaScript code safer. These functions include invariant, warning, assertNever, and more.

Why this library exists?

There are many helpful functions that can make your code safer, such as invariant and assertNever. Many of these functions are available as separate npm packages (for example, invariant). But installing a new package (and most of the time the @types counterpart as well) for each function is not very convenient. Also, those packages generally export their functions as default exports, making VSCode auto-import not work well with them.

This library unifies the functions that make you code safer into a single place. circumspect is a single dependency that has them all. And VSCode auto-import works as expected because all function provided by circumspect are named exports.

Moreover, circumspect is 100% written in TypeScript, so every function is properly typed by default. So, there's no need to install separate @types packages.


Using yarn:

yarn add circumspect

Using npm:

npm install circumspect



invariant(value: unknown, message?: string): asserts value


  • value: unknown is the value that we want to ensure to be truthy.

  • message?: string is an optional error message that will be included in the error that is thrown when the passed value is falsy. This custom error message is displayed only in development. In production, a generic error is shown instead ('Invariant violation') and the message parameter is ignored.

Return value

  • asserts value which means that the function simply narrows the type of value to be truthy and doesn't return anything.


invariant ensures that the specified value is truthy. If that's not the case, it throws an error containing the specified message (in development only). This function properly narrows the type of value by excluding all falsy values (e.g., null and undefined).

invariant should be used whenever we have a value that could potentially be falsy, but we are confident that in a particular circumstance, the value must be truthy. That is, the fact that the value is truthy is an invariant, and if it ever happens that the value is falsy, the invariant has been violated, and thus an error must be thrown.

Since the message argument is completely ignored in production, you might want to strip it from your code completely in production builds. To see how to do that, see the Optimizations section.


declare const user: User | null | undefined;

invariant(user, 'The user is missing!');

user; // If we get here, the type is narrowed to `User`

In this example, the user object can potentially be null or undefined (i.e., falsy). If that's the case, we have an invariant violation, and the invariant function will throw. Otherwise, it simply returns and we know that user actually points to a user object.


warning(value: unknown, message: string): void


  • value: unknown is the value that we want to check. If it is falsy, a warning is issued to the console.

  • message: string is the warning message that will be written to the console.

Return value

  • void the function doesn't return anything.


warning issues a warning to the console with the given message if the specified value is falsy. The warning is issued only in development. In production, this function doesn't do anything.

It should be used whenever we want to issue a development-only warning if some value is falsy, which should help guide developers to fix the non-critical issues being reported in the warning message.

Since warning doesn't do anything in production, you might want to remove calls to this function completely from your code in production builds. To see how to do that, see the Optimizations section.


declare const mode: 'auto' | 'default' | 'slow' | 'fast';

  mode !== 'auto',
  'Mode "auto" has been deprecated. Please use "default" instead.',

In this example, we want to issue a deprecation warning if the mode is 'auto'. To do so, we need to pass a falsy value, that's why we pass mode !== 'auto', which is falsy only when the mode is 'auto'.

In some cases, it makes sense to pass false directly. For example:

declare const languages: Language[];
declare const defaultLanguage: Language;
declare const langName: string;

let lang = languages.find(({ name }) => name === langName);

if (!lang) {
    `Language with name "${langName}" not found. Falling back to the default language.`,

  lang = defaultLanguage;


assertNever(value: never): never


  • value: never is the value after all possible union type variants have been exhausted.

Return value

  • never which means that the function never returns; it just throws an error if ever actually called.


assertNever should be used to ensure that all variants of a union type have been exhausted.

If all union variants of value have been exhausted, there is no compiler error when calling assertNever with value, since value is considered to be of type never at that point, and at run time, we never reach the point of calling assertNever, meaning that no error will be thrown.

However, if not all union variants have been exhaused, then we are calling assertNever with something other than never and so there will be a compiler error saying something like

Argument of type 'x' is not assignable to parameter of type 'never'.

which we can fix by handling the missing variants. You can read more about union exhaustiveness checking and assertNever in the TypeScript docs.


declare const state: 'loading' | 'done' | 'error';

switch (state) {
  case 'loading':
    return <Loading />;
  case 'done':
    return <Done />;
  case 'error':
    return <Error />;

In this example, we handle all possible states inside the switch statement: 'loading', 'done', and 'error'.

However, what if in the future we add another state, such as 'pending'?

The fact that the switch statement is not handling 'pending' would be undetected.

The solution is to have a default case in which we assert that all possible states have been handled.

switch (state) {
    return assertNever(state);

So, when all state variants are handled, we get no compile time error. However, when we add the new 'pending' state, we will get a compiler error saying:

Argument of type 'string' is not assignable to parameter of type 'never'.

We can fix this error by handling the 'pending' state inside the switch.

As you can see from this example, assertNever is especially useful in switch statements where we want to ensure to have handled all possible cases at all times.


nonNull<T>(value: T | null | undefined): value is T


  • value: T | null | undefined is the value that we want to check for not being null or undefined.

Return value

  • value is T which means that the function returns a boolean indicating whether the passed value is neither null nor undefined. This narrows the type of value to T (i.e., excludes null and undefined) when true is returned.


nonNull is a predicate function that checks whether the specified value is non-null, i.e., neither null nor undefined. After calling the function, the type of value is properly narrowed based on whether true or false was returned.

nonNull should be used whenever we have a value that could potentially be null, undefined, or both and we want to check for that and have its type properly narrowed.

The name of this function stems from the NonNullable utility type, which excludes null and undefined from a type.


declare const names: (string | null)[];

const nonNullNames = names.filter(nonNull);

// nonNullNames has type 'string[]'

In this example, we have an array of names that can also include some null elements. We filter the array for all non-null elements and get back a string[].

If we instead used names.filter(x => x !== null), we would get back non-null elements, but the type would still be (string | null)[] since TypeScript sees the function that we pass to filter as just returning a boolean and so no type narrowing occurs.


We recommend using babel-plugin-dev-expression to strip the message argument passed to invariant and to completely remove calls to warning in production.

Basically, in production, babel-plugin-dev-expression replaces

invariant(value, 'Value is falsy!');


if (!value) {

so the message argument is stripped. It also completely removes calls to warning. So, lines like this

warning(value, 'Value is falsy!');

are removed.


Pull requests are very welcome. If you intend to introduce a major change, please open a related issue first in which we can discuss what you would like to change.

Please make sure to update the tests and the README as appropriate.




1 year ago


1 year ago