Promise: basics

a Promise represents an operation that hasn`t completed yet, but is expected in the future

• Promise is a global function

the constructor

• instantiating it without params throws
• expects a function as parameter

simplest promises

• resolve a promise by calling the resolve function given as first parameter
• the resolve function can return a value, that is consumed by the promise.then() callback
• rejecting a promise is done by calling the callback given as 2nd parameter

an asynchronous promise

• can resolve later, also by calling the first callback
• reject it at some later point in time, calling the 2nd callback

test library (mocha here) support for promises

• just returning the promise makes the test library check that the promise resolves

Links

spec A well understandable description of the states a promise can be in.