[JS] JavaScript Beginner (3) Advanced

This section will focus on the following points.

Knowledge points: Multithreading, scopes, closures, this

Look after the top

1. Multithreading

In browsers that do not support H5. Use Concurrent.Thread.js.

In support H5, use webwork.

Before H5, JS is not support multi-threaded. There is the concept of a synchronous pool and an asynchronous pool. When the synchronization pool finishes processing, the asynchronous pool's events, scheduled tasks, and so on are invoked.

The following is a clear distinction between whether JS is multi-threaded execution

C#

Js

    <script>        setinterval (function () {            alert (' 1 ');        }, +);    </script>

Use Concurrent.Thread.js (essentially JS timer function to simulate multithreading)

    <script>        Concurrent.Thread.create (function () {            $ (BTN). Click (func);            function func () {                alert (' message ');            }            var i = 0;            while (1) {                console.log (i);            }        })    </script>

　　

Use Webworker (also cannot manipulate DOM elements)

    <button id= "btn" >Test</button>    <script>        var work = new Worker (' task.js ');        Work.onmessage = function (event) {         //callback function            alert (event.data);        }        Work.postmessage ();                         Call work, you can pass the argument        $ (BTN). Click (func);        function func () {            alert (' message ');        }    </script>task.js:onmessage = func; Called Method function func () {    var i = 0;    while (1) {        console.log (i++);    }}

　　

2. Scope

JS is a function-level scope with internal access to external variables. No block-level scope (if else try catch)

    <script>        if (false) var j = +;         1. Function-level scope  2. Variable Front        alert (j);                       Pop-up undefined        alert (i);                       Error not defined    </script>

The self-executing function executes after declaring the variable.

    <script>        var j = +;        +function func () {          //Here's ~, can also be replaced with +,!, ~ and other unary operator            alert (j);               Eject undefined            var J;                  Variable Front        } ();                        Without this, the parser will assume that function is the beginning of a functional declaration, and the latter () will result in a syntax error. When the function is preceded by a ~, it becomes a functional expression, and a function expression is added after another () becomes a function that executes immediately.    </script>

　

　

3. Closures

Closures are changing the scope of variables. Closures can create memory problems that cannot be released.

        function func () {            var j = Ten;                 Functions outside the function cannot be accessed to J            return function () {        //by means of closures to allow access to J                return J;            }        }        var k = func () ();        Alert (k);

　

　

4.this

Who calls, this refers to WHO. When a closure exists, be aware of the caller this is pointing to.

    <script>        this.m = ten;        var obj = {            m:1            , Func:function () {                               alert (THIS.M);            }            , Bb:function () {                 //closure function                return func tion () {                    alert (THIS.M);}}        }        Obj.func ();                             This refers to obj        obj.bb () ();                             End this means window    </script>

　　

Accessories: Concurrent.Thread.js

This section is also just a call, the content may be wrong, I hope that if you find a mistake, please timely inform.

[JS] JavaScript Beginner (3) Advanced