Understanding Promises

Mike Sherov

Promise.resolve('Principal').then(role => console.log(`${role} Engineer`));

Core UX @ Skillshare

What is a Promise?

Promises are future values!

or a future reason that the value couldn't be gotten!

Why Promises?

  • Avoids Callback hell
  • Allows Composition  of async operations
  • Creates a building block for future async syntax abstractions
  • Provides a unified way of dealing with transaction timing
  • callback: a function to be executed (event handlers)
  • async: code that executes in a future call stack! (or between call stacks!?!?!)
  • Settled vs. Pending:
    • settled: async operation has completed
    • pending: async operation has not completed
  • Resolved vs. Rejected:
    • Resolved (fulfilled): completely successfully
    • Rejected: completed unsuccessfully 

Some Jargon

  • via constructor with "executor" callback
  • via Promise.resolve
  • via Promise.reject
  • anything you return/throw in an async function
  • anything you return/throw in a "then" function

Creating a promise

  • .catch ... this is just sugar for .then(null, errorCallback)
  • .then(successCallback, errorCallback)
    • will call successCallback when promise resolves, or immediately (but still async) if the promise is already resolved
    • will call errorCallback when promise rejects, or immediately (but still async) if the promise is already rejected

using a promise (basic)

  • successCallback receives as it's first argument the resolved value
  • errorCallback receive as it's first argument the rejected reason

using a promise (values & reasons)

  • a resolved promise can be turned into a rejected one if...
    • you return a new rejected Promise in successCallback
    • you throw in a successCallback
  • a rejected promise can be turned into a resolved one if...
    • you don't throw in a errorCallback
    • you don't return a rejected Promise in errorCallback

using a promise (values and reasons)

  • REMEMBER: .then returns a new promise, with a new resolved value (or rejected reason) for future .thens!
  • can avoid this changing what you're chaining off of!
  • if you return a Promise from within .then, the return value of .then is the inner promise's value! tricky

using a promise (chaining)

  • Promise.all: takes in an array of Promises (or values), and will return a resolved promise whose value is an array of the resolved values of the passed in promises if they all resolve, otherwise returns a rejected promise with the reason set to the reason the first promise rejected
  • Promise.race: takes in an array of Promises (or values), and will return a resolved promise whose value is the value of the first promise that resolves, or reject with the first promise that rejects

Promise.all & Promise.race

Understanding Promises

By mikesherov

Understanding Promises

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