How to make Jest wait for all asynchronous code to finish execution before expecting an assertion

When writing unit tests for asynchronous JavaScript code, it is important to ensure that all asynchronous code has finished executing before making any assertions. This is crucial to avoid false positives or false negatives in your test results. In this blog post, we will explore different ways to make Jest wait for all asynchronous code to finish execution before expecting an assertion.

Solution 1: Using Promises

One way to make Jest wait for all asynchronous code to finish execution is by returning a Promise from your test function. Jest will automatically wait for the Promise to resolve before considering the test complete.

Here’s an example:

test('example test', () => {
  return new Promise((resolve) => {
    // Perform asynchronous operations here
    setTimeout(() => {
      // Assertion code here
      expect(2 + 2).toBe(4);
      resolve();
    }, 1000);
  });
});

In this example, we create a Promise and perform our asynchronous operations inside the Promise’s executor function. Once the asynchronous operations are complete, we make our assertion and resolve the Promise to signal to Jest that the test is complete.

Solution 2: Using async/await

Another way to make Jest wait for all asynchronous code to finish execution is by using the async/await syntax. This allows us to write asynchronous code in a synchronous style.

Here’s an example:

test('example test', async () => {
  // Perform asynchronous operations here
  await new Promise((resolve) => {
    setTimeout(() => {
      // Assertion code here
      expect(2 + 2).toBe(4);
      resolve();
    }, 1000);
  });
});

In this example, we mark the test function as async and use the await keyword to wait for the Promise to resolve. This ensures that Jest will wait for all asynchronous code to finish executing before considering the test complete.

Solution 3: Using the done callback

If you prefer not to use Promises or async/await, you can use the done callback provided by Jest to make it wait for asynchronous code to finish execution.

Here’s an example:

test('example test', (done) => {
  // Perform asynchronous operations here
  setTimeout(() => {
    // Assertion code here
    expect(2 + 2).toBe(4);
    done();
  }, 1000);
});

In this example, we pass a done callback to the test function. Once the asynchronous operations are complete, we call the done callback to signal to Jest that the test is complete.

These are three different ways to make Jest wait for all asynchronous code to finish execution before expecting an assertion. Choose the approach that best suits your needs and coding style.