Jest - Testing Asychronous Code

Testing Asychronous Code

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const add = (a, b) => {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            if (a < 0 || b < 0) {
                return reject('Numbers must be non-negative')
            }
            resolve(a + b);
        }, 2000)
    })
}

The callback function for the first test case accepts a done parameter. This lets Jest know that the test function contains asynchronous code. Jest won’t determine if the test passed or failed until done is called. In the example below, then is called to run some code after the numbers are added. This is where the assertion is added and it’s where done is called.

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test('Should add two numbers', (done)=>{
  add(2,3).then((sum)=>{
    expect(sum).toBe(5)
    done()
  })
})

Using async/await is also possible.

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test('Should add two numbers async/await', async()=>{
  const sum = await add(2,3)
  expect(sum).toBe(5)
})

Ref : The Complete Node.js Dev Course by Andrew Mead