JavaScript Promise Cheat Sheet — Handing Promise Results

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JavaScript promises lets us add non-IO-blocking, asynchronous code easily into our JavaScript app sequentially.

Therefore, they’re used everywhere.

In this article, we’ll look at how to use promises in our JavaScript code.

We can create a promise that resolves with Promise.resolve .

For instance, we can write:

Promise.resolve(1)
  .then((val) => console.log(val))

Then val is 1 since we passed in 1 to Promise.resolve .

We can create a promise that resolves with Promise.reject.

For instance, we can write:

Promise.reject('error')
  .catch((err) => console.log(err))

Then err is 'error' since the catch callback catches the reason value of the rejected promise.

We can handle errors that are raised by rejected promises with the Promise.prototype.catch instance method.

For instance, we can write:

Promise.reject('error')
  .catch((err) => console.log(err))

Then err is 'error' since the catch callback catches the reason value of the rejected promise.

This lets us handle the value from the first rejected promises.

We can use the Promise.prototype.then method to get value from resolved promises.

For instance, we can write:

Promise.resolve(1)
  .then(() => Promise.resolve(2))
  .then((val) => console.log(val))

We call then with a callback with a parameter to get the resolved value from the previously resolved promise.

So val is 2.

We can use the Promise.prototype.finally method to run code regardless of what happens to the promises.

For instance, if we have:

Promise.reject('error')
  .catch((err) => console.log(err))
  .finally(() => console.log('finally'))

Then we see 'error' and 'finally' logged since the finally callback runs regardless of whether a promise is resolved or rejected.

We can use promises to make writing async JavaScript code easy.

By John Au-Yeung