How to implement asynchronous in javascript

How to implement asynchronous JavaScript: 1. Use the setTimeout method; 2. Use the setImmediate method; 3. Use the requestAnimationFrame method; 4. Monitor the new Image loading status; 5. Monitor the script loading status method.

The operating environment of this tutorial: Windows 7 system, JavaScript version 1.8.5, DELL G3 computer.

Javascript method to implement asynchronousness:

1. setTimeout: This is the simplest

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: Asynchronous implementation is achieved by loading an image in the data:image data format and listening to the loading status.

Although some browsers do not support the data:image image data format, they can still trigger their onerror state to achieve asynchronous implementation. However, for IE8 and below, for 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 the script loading status

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

Although some browsers do not support scripts 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);

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