Asynchronous JavaScript Cheat Sheet

Promise basics

• 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.

Creating promises

• 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.
• If you put a fulfilled promise into a fulfilled promise, they will collapse into one.

// 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));

Handling promises

• Promise.prototype.then() accepts two optional arguments (onFulfilled, onRejected).

• Promise.prototype.then() will call onFulfilled once the promise is fulfilled.

• Promise.prototype.then() will call onRejected if the promise is rejected.

• Promise.prototype.then() passes errors through if onRejected in undefined.

• Promise.prototype.catch() accepts one argument (onRejected).

• Promise.prototype.catch() behaves like Promise.prototype.then() when onFulfilled is omitted.

• Promise.prototype.catch() passes fulfilled values through.

• Promise.prototype.finally() accepts one argument (onFinally).

• Promise.prototype.finally() calls onFinally with no arguments once any outcome is available.

• Promise.prototype.finally() passes through input promise.

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
  );

• All three of the above methods will not be executed at least until the next tick, even for promises that already have an outcome.

Combining promises

• Promise.all() turns an array of promises into a promise of an array.
• If any promise is rejected, the error will pass through.
• Promise.race() passes through the first settled promise.

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
  });

async/await

• Calling an async function always results in a promise.
• (async () => value)() will resolve to value.
• (async () => throw err)() will reject with an error.
• await waits for a promise to be fulfilled and returns its value.
• await can only be used in async functions.
• await also accepts non-promise values.
• await always waits at least until the next tick before resolving, even when waiting already fulfilled promises or non-promise values.

async () => {
  try {
    let val = await promisedValue();
    // Do stuff here
  } catch (err) {
    // Handle error
  }
}