Promise() constructor

The Promise constructor is primarily used to wrap functions that do not already support promises.

Syntax

new Promise(executor)

Parameters

executor
A function to be executed by the constructor, during the process of constructing the new Promise object. The executor is custom code that ties an outcome to a promise. You, the programmer, write the executor. The signature of this function is expected to be:
function(resolutionFunc, rejectionFunc){
    // typically, some asynchronous operation.
}

At the time when the constructor generates the new Promise object, it also generates a corresponding pair of functions for resolutionFunc and rejectionFunc; these are "tethered" to the Promise object. Therefore, the code within the executor has the opportunity to perform some operation and then reflect the operation's outcome (If the value is not another Promise object) as either "fulfilled" or "rejected" by terminating with an invocation of either the resolutionFunc or the rejectionFunc, respectively.

The executor has no meaningful return value. It communicates via the side-effect caused by resolutionFunc or rejectionFunc. The side-effect is that the Promise object becomes "resolved."

Typically, it works like this: The operation within executor is asynchronous and provides a callback. The callback is defined within the executor code. The callback terminates by invoking resolutionFunc. The invocation of resolutionFunc includes a value parameter. The value is passed back to the tethered Promise object. The Promise object (asynchronously) invokes any .then() associated with it. The value received by .then() is passed to the invocation of handleFulfilled as an input parameter (See "Chained Promises" section).

The executor might also include a try{} catch() block that invokes rejectionFunc upon error.

The signatures of these two functions are simple, they accept a single parameter of any type. Of course, the actual names of these functions can be whatever is desired, i.e. they are named as the parameters of executor. Each function is used by calling it when appropriate.

resolutionFunc(value) // call on fulfilled
rejectionFunc(reason) // call on rejected

The returned value can be another promise object, in which case the promise gets dynamically inserted into the chain.

Return value

When called via new, the Promise constructor returns a promise object. The promise object will become "resolved" when either of the functions resolutionFunc or rejectionFunc are invoked. Note that if you call resolutionFunc or rejectionFunc and pass another Promise object as an argument, you can say that it is "resolved", but still cannot be said to be "settled".

Examples

Creating a new Promise

A Promise object is created using the new keyword and its constructor. This constructor takes a function, called the "executor function", as its parameter. This function should take two functions as parameters. The first of these functions (resolve) is called when the asynchronous task completes successfully and returns the results of the task as a value. The second (reject) is called when the task fails, and returns the reason for failure, which is typically an error object.

const myFirstPromise = new Promise((resolve, reject) => {
  // do something asynchronous which eventually calls either:
  //
  //   resolve(someValue)        // fulfilled
  // or
  //   reject("failure reason")  // rejected
});

Making functions return a Promise

To provide a function with promise functionality, have it return a promise:

function myAsyncFunction(url) {
  return new Promise((resolve, reject) => {
    const xhr = new XMLHttpRequest()
    xhr.open("GET", url)
    xhr.onload = () => resolve(xhr.responseText)
    xhr.onerror = () => reject(xhr.statusText)
    xhr.send()
  });
}

Specifications

Specification
ECMAScript Language Specification (ECMAScript)
# sec-promise-constructor

Browser compatibility

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See also