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Asynchronous JavaScript Cheat Sheet

Submitted by: @import:30-seconds-of-code·Mar 2, 2026·

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javascriptcheatsheetasynchronous

Problem

Promises start in a pending state, neither fulfilled or rejected.
When the operation is completed, a promise will become fulfilled with a value.
If the operation fails, a promise will get rejected with an error.

@Quick refresher

The function passed to the Promise constructor will execute synchronously.

Solution

// Resolving with a value, rejecting with an error
new Promise((resolve, reject) => {
  performOperation((err, val) => {
    if (err) reject(err);
    else resolve(val);
  });
});

// Resolving without value, no need for reject
const delay = (ms) => new Promise(resolve => setTimeout(resolve, ms));

If the operation fails, a promise will get rejected with an error.

@Quick refresher

The function passed to the Promise constructor will execute synchronously.
Use resolve() or reject() to create promises from values.
Promise.resolve(val) will fulfill the promise with val.
Promise.reject(err) will reject the promise with err.

Code Snippets

// Resolving with a value, rejecting with an error
new Promise((resolve, reject) => {
  performOperation((err, val) => {
    if (err) reject(err);
    else resolve(val);
  });
});

// Resolving without value, no need for reject
const delay = (ms) => new Promise(resolve => setTimeout(resolve, ms));

promisedOperation()
  .then(
    val => value + 1,   // Called once the promise is fulfilled
    err => {            // Called if the promise is rejected
      if (err === someKnownErr) return defaultVal;
      else throw err;
    }
  )
  .catch(
    err => console.log(err); // Called if the promise is rejected
  )
  .finally(
    () => console.log('Done'); // Called once any outcome is available
  );

Promise
  .all([ p1, p2, p3 ])
  .then(([ v1, v2, v3 ]) => {
    // Values always correspond to the order of promises,
    // not the order they resolved in (i.e. v1 corresponds to p1)
  });

Promise
  .race([ p1, p2, p3 ])
  .then(val => {
    // val will take the value of the first resolved promise
  });

Context

From 30-seconds-of-code: async-cheatsheet

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