How Can I Turn `setTimeout` into a Promise in JavaScript?

Turning setTimeout into a Promise

Overview

In this article, we'll explore how to transform setTimeout, a function that executes a callback after a specified delay, into a Promise that can be used to handle asynchronous operations.

Native Promises

Modern JavaScript supports native Promises that provide a convenient way to represent asynchronous operations. To convert setTimeout into a Promise, we can wrap it in a Promise constructor and pass the resolve function as the callback:

function later(delay) {
    return new Promise((resolve) => {
        setTimeout(resolve, delay);
    });
}

This function will return a Promise that resolves after the specified delay.

Customized Promises

If you prefer to create your own Promise implementation, you can define a promise prototype and use it as follows:

function setTimeoutReturnPromise() {
    function promise() { }
    
    promise.prototype.then = function() {
        console.log('timed out');
    };

    setTimeout(() => {
        return this;  // Return the promise object
    }, 2000);

    return new promise();
}

Cancellable Promise

To create a cancellable Promise, we can wrap setTimeout in a custom object that provides a cancel method:

const later = (delay, value) => {
    let timer = 0;
    let reject = null;
    const promise = new Promise((resolve, _reject) => {
        reject = _reject;
        timer = setTimeout(resolve, delay, value);
    });
    
    return {
        get promise() { return promise; },
        cancel() {
            if (timer) {
                clearTimeout(timer);
                timer = 0;
                reject();
                reject = null;
            }
        }
    };
};

This object returns a Promise and a cancel method that can be used to terminate the operation.

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