Testing Asynchronous Code

Testing Asynchronous Code

It's common in JavaScript for code to run asynchronously. When you have code that runs asynchronously, Jest needs to know when the code it is testing has completed, before it can move on to another test. Jest has several ways to handle this.

Callbacks

The most common asynchronous pattern is callbacks.

For example, let's say that you have a fetchData(callback) function that fetches some data and calls callback(data) when it is complete. You want to test that this returned data is just the string 'peanut butter'.

By default, Jest tests complete once they reach the end of their execution. That means this test will not work as intended:

// Don't do this!
test('the data is peanut butter', () => {
  function callback(data) {
    expect(data).toBe('peanut butter');
  }

  fet	          			          		
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