How can I create an Asynchronous function in Javascript?

Question

You cannot make a truly custom asynchronous function. You'll eventually have to leverage on a technology provided natively, such as:

• setInterval
• setTimeout
• requestAnimationFrame
• XMLHttpRequest
• WebSocket
• Worker
• Some HTML5 APIs such as the File API, Web Database API
• Technologies that support onload
• ... many others

In fact, for the animation jQuery uses setInterval.

You can use a timer:

setTimeout( yourFn, 0 );

(where yourFn is a reference to your function)

or, with Lodash:

_.defer( yourFn );

Defers invoking the func until the current call stack has cleared. Any additional arguments are provided to func when it's invoked.

here you have simple solution (other write about it) http://www.benlesh.com/2012/05/calling-javascript-function.html

And here you have above ready solution:

function async(your_function, callback) {
    setTimeout(function() {
        your_function();
        if (callback) {callback();}
    }, 0);
}

TEST 1 (may output '1 x 2 3' or '1 2 x 3' or '1 2 3 x'):

console.log(1);
async(function() {console.log('x')}, null);
console.log(2);
console.log(3);

TEST 2 (will always output 'x 1'):

async(function() {console.log('x');}, function() {console.log(1);});

This function is executed with timeout 0 - it will simulate asynchronous task

Here is a function that takes in another function and outputs a version that runs async.

var async = function (func) {
  return function () {
    var args = arguments;
    setTimeout(function () {
      func.apply(this, args);
    }, 0);
  };
};

It is used as a simple way to make an async function:

var anyncFunction = async(function (callback) {
    doSomething();
    callback();
});

This is different from @fider's answer because the function itself has its own structure (no callback added on, it's already in the function) and also because it creates a new function that can be used.

Edit: I totally misunderstood the question. In the browser, I would use setTimeout. If it was important that it ran in another thread, I would use Web Workers.

This page walks you through the basics of creating an async javascript function.

Since ES2017, asynchronous javacript functions are much easier to write. You should also read more on Promises.