new Promise

new Promise(function(function resolve, function reject) resolver) -> Promise

Create a new promise. The passed in function will receive functions resolve and reject as its arguments which can be called to seal the fate of the created promise.

Note: See explicit construction anti-pattern before creating promises yourself

Example:

function ajaxGetAsync(url) {
    return new Promise(function (resolve, reject) {
        var xhr = new XMLHttpRequest;
        xhr.addEventListener("error", reject);
        xhr.addEventListener("load", resolve);
        xhr.open("GET", url);
        xhr.send(null);
    });
}

If you pass a promise object to the resolve function, the created promise will follow the state of that promise.

To make sure a function that returns a promise is following the implicit but critically important contract of promises, you can start a function with new Promise if you cannot start a chain immediately:

function getConnection(urlString) {
    return new Promise(function(resolve) {
        //Without new Promise, this throwing will throw an actual exception
        var params = parse(urlString);
        resolve(getAdapter(params).getConnection());
    });
}

The above ensures getConnection fulfills the contract of a promise-returning function of never throwing a synchronous exception. Also see Promise.try and Promise.method

The resolver is called synchronously (the following is for documentation purposes and not idiomatic code):

function getPromiseResolveFn() {
    var res;
    new Promise(function (resolve) {
        res = resolve;
    });
    // res is guaranteed to be set
    return res;
}