The async keyword is used to write functions that handle asynchronous actions. We wrap our asynchronous logic inside a function prepended with the async keyword. Then, we invoke that function.

async function myFunc() { // Function body here }; myFunc();

We’ll be using async function declarations throughout this lesson, but we can also create async function expressions:

const myFunc = async () => { // Function body here }; myFunc();

async functions always return a promise. This means we can use traditional promise syntax, like .then() and .catch with our async functions. An async function will return in one of three ways:

  • If there’s nothing returned from the function, it will return a promise with a resolved value of undefined.
  • If there’s a non-promise value returned from the function, it will return a promise resolved to that value.
  • If a promise is returned from the function, it will simply return that promise
async function fivePromise() { return 5; } fivePromise() .then(resolvedValue => { console.log(resolvedValue); }) // Prints 5

In the example above, even though we return 5 inside the function body, what’s actually returned when we invoke fivePromise() is a promise with a resolved value of 5.

Let’s write an async function!



We provided a function withConstructor() which takes in a number. If the number is 0, it returns a promise that resolves to the string 'zero'. If the number is not 0, it returns a promise that resolves to the string 'not zero'. Take a moment to understand this function and the code that follows. When you’re ready to run it, type node app.js in to the terminal and press enter.


Write an async function, withAsync() which reproduces the functionality of withConstructor(). Though your function will return a promise, it should not construct the promise using the new keyword. Instead, it should rely on the fact that an async function automatically returns a promise.

When you’re ready, check your work to move on to the next step.


Now test your code! Uncomment the test code we wrote at the bottom of app.js. In the terminal, type node app.js and press enter to execute the code.

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