Learning Redux

Callbacks

This is the oldest and most common way to deal with asynchronous code in JavaScript. You have a function that accepts a callback function as an argument; for example, fetchPosts(callback). When the asynchronous request finishes, the callback function is executed:

function fetchPosts (callback) {  // usually, we would fetch posts in an asynchronous request here  // in this example, we simply execute the callback  callback([ { title: 'test' }, { title: 'hello world' } ])}

By default, Jest marks a test as complete when it reaches the end of the test function, which means that the following test will finish too early and not execute the matcher:

test('this finishes too early', () => {  fetchPosts(posts => expect(posts).toContain({ title: ...