promise-chain-settled

promise-chain-settled

Provides a way of knowing when a promise chain is settled. Useful for testing.

Installation

npm install --save promise-chain-settled

or available on JSDelivr at "https://cdn.jsdelivr.net/npm/promise-chain-settled@1".

API

First you need to wrap the promise with wrap(promise).

This returns an object with the following properties:

numPending(): number

This returns the number of promises in the chain that are currently pending. It can increase at any time if new items are added.

const promise = new Promise(() => {});
const wrapped = wrap(promise);
console.log(wrapped.numPending()); // 1
promise.then(() => {});
console.log(wrapped.numPending()); // 2

isChainSettled(): boolean

Returns true when numPending() === 0 meaning everything in the chain is settled. This can change at any time if new items are added.

const promise = Promise.resolve();
const wrapped = wrap(promise);
console.log(wrapped.isChainSettled()); // false
await promise;
console.log(wrapped.isChainSettled()); // true
promise.then(() => {});
console.log(wrapped.isChainSettled()); // false

whenChainSettled(): Promise

This returns a promise that resolves the next time isChainSettled() goes from false to true.

onChange(listener: () => void): { remove: () => void }

This lets you add a listener that will be invoked whenever numPending() changes.

const promise = Promise.resolve();
const wrapped = wrap(promise);
wrapped.onChange(() => {
  // first call: new numPending() 2
  // second call: new numPending() 1
  // third call: new numPending() 0
  console.log('new numPending()', wrapped.numPending());
});
promise.then(() => {});

Example

import { wrap } from 'promise-chain-settled';

const promise = Promise.resolve();

// modify the promise so that we can keep track of everything in the chain
const wrapped = wrap(promise);

promise
  .then(() => {
    return somethingAsync();
  })
  .then(() => {
    return somethingElseAsync();
  })
  .then((something) => {
    console.log('Done');
  });

wrapped.whenChainSettled().then(() => {
  console.log('Chain settled');
});

would log

Done
Chain settled