Creating a (ES6) promise without starting to resolve it

Good question!

The resolver passed to the promise constructor intentionally runs synchronous in order to support this use case:

var deferreds = [];
var p = new Promise(function(resolve, reject){
    deferreds.push({resolve: resolve, reject: reject});

Then, at some later point in time:

 deferreds[0].resolve("Hello"); // resolve the promise with "Hello"

The reason the promise constructor is given is that:

  • Typically (but not always) resolution logic is bound to the creation.
  • The promise constructor is throw safe and converts exceptions to rejections.

Sometimes it doesn’t fit and for that it the resolver runs synchronously. Here is related reading on the topic.

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