programming JavaScript Oct 27, 2015

JavaScript Promises

I read Best Practices for Using Promises in JS (opens new window). Here are some concise notes expanding on this topic.

Let's use bluebird as Promise implementation for node
Use .then().catch() instead of .then(successCb, failCb)

const promise = (shouldResolve) => new Promise((resolve, reject) => {
  if (shouldResolve) {
    resolve();
  } else {
    reject();
  }});

promise(arg)
  .then(() => { /* success */ })
  .catch(ExceptionToCatch, OtherExceptionToCatch, (err) => { /* error */})
  .catch(StrangeExceptionToCatch, (err) => { /* error */})
  .catch((err) => { /* error */}); // default

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To define an exception to catch:

function MyCustomError() {}
MyCustomError.prototype = Object.create(Error.prototype);

promise(arg)
  .then(() => { throw new MyCustomError() })
  .catch(MyCustomError, (err) => {})
  .catch((err) => {});

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Promise.all([promiseA(a), promiseB(b)]) runs two async functions in parallel, but .then(() => {}) callback has no data result argument
Use .spread instead of .then to get this data: .spread((dataA, dataB) => {})
To limit concurrency, Promise.map([lotsOfStuff], promise, { concurrency: 3 }).then(() => {})
See also .reduce and .filter
You can "pipe" your .then functions. If you do this, you should probably avoid using anonymous functions and enjoy more reusability, modularity, testability, readability

readFile(data)
  .then(JSON.parse)
  .then(treatStuff)
  .catch(handleError)

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Don't abuse this thing. It's nice to read, but it breaks the event loop and could introduce race conditions
A better approach is to pipe inside the .then:

readFile(data)
  .then(pipe(JSON.parse, treatStuff))
  .catch(handleError)

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JavaScript Promises

Further reading: