Testing Function

What you'll learn

  • write tests for pure function in Jest
  • use assertion provided by Jest

In last section, we create our own wrapper to test function.

In this section, we will learn to write tests using Jest.

Creating a canary test

Create a file in src folder and name it canary.spec.js with the following content:

test(`canary test`, () => {

Run the following command in your command line:

yarn test

You should be able to see the output similar to below:

PASS src/canary.spec.js
√ canary test (5ms)
Test Suites: 1 passed, 1 total
Tests: 1 passed, 1 total
Snapshots: 0 total
Time: 3.53s

Let's walkthrough what we've just did:

  1. When we execute yarn test in the command line, it will runs the command being configured in scripts.test in package.json, which is react-scripts test.

    "scripts": {
    "start": "react-scripts start",
    "build": "react-scripts build",
    "test": "react-scripts test",

    "eject": "react-scripts eject",
    "start:docs": "styleguidist server",
    "build:docs": "styleguidist build"
  2. react-scripts is the command provided by Create React App. react-scripts test is the command that execute tests in your project. Under the hood, it uses Jest.

  3. By default, Jest will search in src folder any file that matches the pattern *.spec.js, *.spec.jsx, *.test.js, and *.test.jsx and treat them as test files. Since canary.test.js matches the pattern, it will be selected by Jest and executed.

  4. Jest run all the files and report the test result summary in the command line.

  5. Note that Jest will not exit after all the tests are run. Instead, it will keep watching your files and rerun all the tests if any files is edited. This is very handy as you get instant result of your changes.

Now that we verify our test setup is working fine, it's time to write our first test.

Write Test for Pure Function

Let's write a test for pure functions, as they are usually easy to test.

Codes in src/lib folder are utilities functions, some of them pure and some not.

Let's test the camelize function in src/lib/camelize.js.

Create a camelize.spec.js file next to camelize.js with the following contents:

import { camelize } from './camelize';
test(`camelize`, () => {

Let's write a few more test cases:

import { camelize } from './camelize';
test(`camelize`, () => {





And the test is still passing!

If you're like me, you probably will start doubting yourself if everytime goes too well. Let's "pinch ourselves" by introducing bug to our code:

const hyphenPattern = /-(.)/g;
* camel-case a hyphenated string
* @example camelize('background-color') => 'backgroundColor'
export function camelize(stringWithHyphen) {
return stringWithHyphen.replace(hyphenPattern, function (_, character) {
// return character.toUpperCase();

return character;


Now you should be seeing the test fail!

FAIL src/lib/camelize.spec.js
× camelize (13ms)
● camelize
expect(received).toBe(expected) // Object.is equality
Expected: "backgroundColor"
Received: "backgroundcolor"
2 |
3 | test(`camelize`, () => {
> 4 | expect(camelize('background-color')).toBe('backgroundColor');
| ^
5 | expect(camelize('border-top-color')).toBe('borderTopColor');
6 | expect(camelize('margin')).toBe('margin');
7 | expect(camelize('marginLeft')).toBe('marginLeft');
at Object.<anonymous>.test (src/lib/camelize.spec.js:4:40)

Let's revert back the changes and you should be seeing the test pass again.


  1. Write tests for the map function in src/lib/array.js.
  2. (Optional) Write tests for the flattenArray function in src/lib/array.js.


Let's write a test for pick function in src/lib/object.js:

import { pick } from './object';
test(`pick`, () => {
a: 'A',
b: 'B',
a: 'A',

And the test fails!

● pick
expect(received).toBe(expected) // Object.is equality
If it should pass with deep equality, replace "toBe" with "toStrictEqual"
Expected: {"a": "A"}
Received: serializes to the same string
10 | ['a']
11 | )
> 12 | ).toBe({
| ^
13 | a: 'A',
14 | });
15 | });
at Object.<anonymous> (src/lib/object.spec.js:12:5)

If you console.log the result of the pick call, you would see {a: 'A'}. Why does it fail?

If you read the error message above, you may already know why. This is because .toBe assertion compare value using Object.is, which check for identity equality if the comparison is between two objects.

Fortunately, the error message also tell us how to fix that: just change the assertion from .toBe to toStrictEqual, and the test will pass.

The test teaches us something new: we need to be careful that our comparisons is about two objects or two primitive value. But how do we know other assertions that are available in Jest?

Two things to help you discover:

  1. Scan through Jest expect documentation.
  2. Enable Intellisense to provide suggestions.

Scanning through the docs will be left as an exercise for you. I will show you how to enable Intellisense.

To enable Intellisense, you need to install @types/jest (TypeScript definition for Jest) by running the following commands:

yarn add -D @types/jest

Once the installation completed, Intellisense will shows you the available assertion as shown below:

Intellisense showing available assertions in VS Code

Now you're be able to explore available assertions.


  1. Install Jest type definition to enable Intellisense.
  2. Write tests for the omit function in src/lib/object.js.

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