Promise.factory

index.md

The ES6 Promise specification contains the very useful constructor for creating a promise, like so:

var promise = new Promise(function(resolve, reject){
  resolve("hi");
});

promise.then(function(message){
  console.log(message);
});

But often you need to create a promise several times, with slightly different parameters, and this ends up as a small wrapper function around the Promise constructor:

function greet(name){
  return new Promise(function(resolve, reject){
    resolve("hi "+name);
  });
}

greet("Pete").then(function(message){
  console.log(message);
});

It would therefore be useful if this was built into the Promise object as a factory, so you could instead do:

var greet = Promise.factory(function(name, resolve, reject){
  resolve("hi "+name);
});

greet("Pete").then(function(message){
  console.log(message);
});

The factory takes any number of parameters and adds the promise callbacks (resolve and reject) to the end of the argument list. It can be defined like this:

Promise.factory = function(factory){
  return function(...args){
    return new Promise(function(...cbs){
      return factory(...args, ...cbs);
    });
  }
}

This can be used for, for example, ajax:

var getData = Promise.factory(function(params, resolve, reject){
  $.getJSON("/path/to/endpoint", params, function(data){
    resolve(data);
  });
});

Promise.all([
  getData({key: "value1"}),
  getData({key: "value2"}),
  getData({key: "value3"})
]).then(function(result){
  console.log(result[0], result[1], result[2]);
});