What are the ways to implement asynchronousness in javascript?

Method: 1. Use setTimeout; 2. Use setImmediate; 3. Use requestAnimationFrame; 4. Realize by monitoring the "new Image" loading status; 5. Realize by monitoring the script loading status; 6. Use Message etc.

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

Native javascript way to achieve asynchronous implementation:

1. setTimeout: This is the simplest one

setTimeout( function() {
    console.log(1);
});
console.log(2);

2. setImmediate: a new function added by IE10, specifically used to liberate the ui thread. IE10 and below and other browsers do not support

setImmediate(function(){
    console.log(1);
});
console.log(2);

3. requestAnimationFrame: a new product in the HTML5/CSS3 era, specially used for animation. Low-level browsers do not support

var asynByAniFrame = (function(){
    var _window = window,
    frame = _window.requestAnimationFrame
            || _window.webkitRequestAnimationFrame
            || _window.mozRequestAnimationFrame
            || _window.oRequestAnimationFrame
            || _window.msRequestAnimationFrame;
    return function( callback ) { frame( callback ) };
})();

asynByAniFrame(function(){
    console.log(1);
})
console.log(2);

4. Monitoring the new Image loading status: by loading an image in the data:image data format and listening to the loading status to achieve asynchronous .

Although some browsers do not support the data:image image data format, they can still trigger their onerror state to achieve asynchronous implementation. However, IE8 and below do not support images in the data:image data format. , onerror is executed synchronously

function asynByImg( callback ) {
    var img = new Image();
    img.onload = img.onerror = img.onreadystatechange = function() {
        img = img.onload = img.onerror = img.onreadystatechange = null;
        callback(); 
    }
    img.src = "data:image/png,";
}
asynByImg(function(){
    console.log(1);
});
console.log(2);

5. Monitor script loading status

The principle is the same as new Image. Generate a data:text/javascript script and monitor its loading status to implement asynchronously.

Although some browsers do not support scripts with data in the data:text/javascript format, they can still trigger their onerror and onreadystatechange events to achieve asynchronous implementation.

var asynByScript = (function() {
    var _document = document,
        _body = _document.body,
        _src = "data:text/javascript,",
        //异步队列
        queue = [];
    return function( callback ) {
            var script = _document.createElement("script");
            script.src  = _src;
            //添加到队列
            queue[ queue.length ] = callback;
            script.onload = script.onerror = script.onreadystatechange = function () {
                script.onload = script.onerror = script.onreadystatechange = null;
                _body.removeChild( script );
                script = null;
                //执行并删除队列中的第一个
                queue.shift()();
            };
            _body.appendChild( script );
        }

    })();

asynByScript( function() {
    console.log(1);
} );
console.log(2);

6. Message: HTML5 new skill, implemented by listening to the window.onmessage event, and then postMessage sends the message, triggering the onmessage event to achieve asynchronous

var asynByMessage = (function() {
        //异步队列
        var queue = [];
        window.addEventListener('message', function (e) {
            //只响应asynByMessage的召唤
            if ( e.data === 'asynByMessage' ) {
                e.stopPropagation();
                if ( queue.length ) {
                    //执行并删除队列中的第一个
                    queue.shift()();
                }
            }
        }, true);
        return function( callback ) {
            //添加到队列
            queue[ queue.length ] = callback;
            window.postMessage('asynByMessage', '*');
        };
    })();

asynByMessage(function() {
    console.log(1);
});
console.log(2);

7. Promise: ES6’s new skill, with asynchronous nature

var asynByPromise = (function() {
        var promise = Promise.resolve({
                then : function( callback ) {
                    callback();
                }
            });
        return function( callback ) {
                    promise.then(function(){
                        callback();
                    })
                };
    })();
asynByPromise(function() {
    console.log(1);
});
console.log(2);

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