Promise: creation

A promise can be created in multiple ways

creating a promise fails when

• using Promise as a function
• no parameter is passed
• passing a non-callable throws too

most commonly Promises get created using the constructor

• by passing a resolve function to it
• by passing a resolve and a reject function to it

extending a Promise

• using class X extends Promise{} is possible
• must call super() in the constructor if it wants to inherit/specialize the behavior

Promise.all() returns a promise that resolves when all given promises resolve

• returns all results
• is rejected if one rejects

Promise.race() returns the first settled promise

• if it resolves first, the promises resolves
• if one of the given promises rejects first, the returned promise is rejected

Promise.resolve() returns a resolving promise

• if no value given, it resolves with undefined
• resolves with the given value

Promise.reject() returns a rejecting promise

• if no value given, it rejects with undefined
• the parameter passed to reject() can be used in the .catch()

Links

spec Describing the promise constructor.
spec How `Promise.all()` is specified.
mdn docs Documenting `Promise.all()`.
spec How `Promise.race()` is specified.
mdn docs Documenting `Promise.race()`.
spec How `Promise.resolve()` is specified.
mdn docs Documenting `Promise.resolve()`.
spec How `Promise.resolve()` is specified.
mdn docs Documenting `Promise.reject()`.