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javascriptconstructorpromise
Jessica
Jessica

Reputation: 2425

"Resolve" functions in JavaScript promises

I am reading this documentation about how to use promises, and frequently "resolve" and "reject" are passed in as arguments to the Promise constructor, even though nobody ever defined the "resolve" or "reject" functions. How is that possible? Don't we have to define functions before using them?

Here's an example: (source: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise#Browser_compatibility)

var p1 = new Promise(
        // The resolver function is called with the ability to resolve or
        // reject the promise
        function(resolve, reject) {
            log.insertAdjacentHTML('beforeend', thisPromiseCount +
                ') Promise started (<small>Async code started</small>)<br/>');
            // This only is an example to create asynchronism
            window.setTimeout(
                function() {
                    // We fulfill the promise !
                    resolve(thisPromiseCount);
                }, Math.random() * 2000 + 1000);
        });

Upvotes: 1

Views: 125

Answers (1)

Bergi
Bergi

Reputation: 664503

They are not passed in as arguments to the Promise constructor.

They are passed as arguments by the Promise constructor into your resolver callback function that declares them as parameters.

This is similar to the parameters of other callbacks, for example

array.sort(function(a, b) { … })
//                  ^  ^
array.map(function(element, index) { … })
//                 ^^^^^^^  ^^^^^

only that the values are functions in the case of the Promise constructor callback.

Upvotes: 1

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