ToBeType

A Jest matcher that allows you to test the expected type of a value.

Usage

Use it like any other matcher. For example:

expect("").toBeType("string");
expect({}).toBeType("object");
expect(1).toBeType("number");
expect([]).toBeType("array");
expect(() => {}).toBeType("function");

// also works with Promises
expect(Promise.resolve([])).resolves.toBeType("array");

It's that easy, enjoy!

Why?

Have you ever been frustrated doing this for every test when you just want to check the type of a value?

	expect(typeof foo).toBe("string");

Or, worse, if you're testing the result of a Promise?

	expect(fooPromise
		.then(data => typeof data))
		.resolves.toBe("object");

It's not hard to write this boilerplate but it is annoying. Strangely Jest specifically doesn't include any easy shorthand for this.

Well it does now.

Installation

Simple install to your project like so:

npm i jest-tobetype --save-dev

Then include in your tests either in the test file you want or in the setup files for Jest.

The simplest way is:

import toBeType from "jest-tobetype";
expect.extend(toBeType);

This is probably all you'll need to do if you're not doing anything special but if you want more options - read one.

If you have multiple extensions you are doing you may want to just import the function directly, eg:

import {toBeType} from "jest-tobetype";
expect.extend({
	toBeType,
	someOtherThing,
	// and so on
});

and if you have a need for it you can also do this:

import {extend} from "jest-tobetype";
extend(expect);

Though that's there mostly just because I like functional programming.

(Note: if you use the setup files make sure to extend in setupTestFrameworkScriptFile as extend is not available in setupFiles).