Mastering Jest: Day 3 — Testing Asynchronous Code

Introduction

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Welcome to Day 3 of our Jest journey! So far, we’ve covered the basics of writing and structuring test cases. Today, we’ll dive into testing asynchronous code. Asynchronous operations, such as API calls and timers, are common in modern JavaScript applications. Testing these operations can be tricky, but Jest provides several powerful tools to handle them effectively.

Asynchronous Testing Basics

Asynchronous tests are essential when your code includes operations that don’t complete immediately. These could be callbacks, promises, or async/await functions. Jest offers a variety of ways to handle these scenarios.

Testing Callbacks

Let’s start with a simple example: testing a function that uses a callback. Consider the following asynchronous function:

// fetchData.js
function fetchData(callback) {
  setTimeout(() => {
    callback('peanut butter');
  }, 1000);
}

module.exports = fetchData;

To test this function, we need to ensure that the callback is executed with the expected value:

// fetchData.test.js
const fetchData = require('./fetchData');

test('the data is peanut butter', done => {
  function callback(data) {
    try {…