前端常用的库和实用技术之JavaScript高级技巧

javascript高级技巧

变量作用域和闭包

<!DOCTYPE html>

<html lang="en">

<head>

    <meta charset="UTF-8">

    <meta name="viewport" content="width=device-width, initial-scale=1.0">

    <meta http-equiv="X-UA-Compatible" content="ie=edge">

    <title>Document</title>

</head>

<body>

    <script>

        // js是函数级作用域，在内部的变量内部都能访问，外部不能访问内部的，内部能访问外部的

        test();

        var j;

        j = 1000;

        function test(){

            if(false){

                var i=10;

            }else{

                var t =100;

            }

            console.log('bbb',t);

            console.log('cccc',j);

        }

        alert('....',t);

    </script>

    <script>

        var j = 100;

        ~(function test(){

            console.log(j);

        })();

        //100

        //-1

    </script>

    <script>

        var j =100;

        function test(){

            var j;

            alert(j);//拿不到外面的J，里面的j值，war在前面

            j=10

        }

        //执行test()根本取不到

        test()

    </script>

    <script>

        //闭包：闭包就是拿到本不属于它的东西

        var j = 100;

        function test(){

            var j;

            j = 10;

            var k = 666;

            return function(){

                return k;

            }

            alert(j);

        }

        var t = test()();

        alert(t);

    </script>

</body>

</html>

this指针的使用

<!DOCTYPE html>

<html lang="en">

<head>

    <meta charset="UTF-8">

    <meta name="viewport" content="width=device-width, initial-scale=1.0">

    <meta http-equiv="X-UA-Compatible" content="ie=edge">

    <title>Document</title>

</head>

<body>

    <script>

        // window.m = 100;

        // // this指针的使用

        // function test(){

        //     alert(this.m);

        // }

        // window.test();

        this.m = 1000;

        var obj = {

            m:100,

            test:function(){

                alert(this.m);

                return function(){

                    alert(this.m);//

                }

            }

        }

        // 谁调指向谁

        var t = obj.test()

        window.t();

        //上面等同于

    //    (obj.test()) ();

    </script>

</body>

</html>

<script>

        this.a = 1000;

        function test(){

            this.a = 1;

        }

        test.prototype.geta = function(){

            return this.a;

        }

        var p = new test;

        console.log(p.geta);

    </script>

    <script>

        function test(){

            this.a = 1;

        }

        test.prototype.a = 100;

        var p = new test;

        console.log(p);

    </script>

<!DOCTYPE html>

<html lang="en">

<head>

    <meta charset="UTF-8">

    <meta name="viewport" content="width=device-width, initial-scale=1.0">

    <meta http-equiv="X-UA-Compatible" content="ie=edge">

    <title>Document</title>

</head>

<body>

    <input type="button" id="test" value="test" style="color:red;">

    <script>

        var style = {

            color:"green"

        }

        window.test();

        document.getElementById("test").click = test;

        function test(){

            alert(this.style.color);

        }

    </script>

</body>

</html>

按值传递和按引用传递

<!DOCTYPE html>

<html lang="en">

<head>

    <meta charset="UTF-8">

    <meta name="viewport" content="width=device-width, initial-scale=1.0">

    <meta http-equiv="X-UA-Compatible" content="ie=edge">

    <title>Document</title>

</head>

<body>

    <script>

        function test(num){

            //这里对num 创建了一个内存的副本

            var num = num+1;

            return num;

        }

        var num = 1;

        alert(test(num));

        alert(num);

    </script>

    <script>

    function test(obj){

        obj.age = "20"

        console.log('内部obj',obj);

    }

    var obj = {

        name:'xiaoming'

    }

    test(obj);

    console.log('外部的..',obj);

    </script>

</body>

</html>

by上面的例子其实不够经典也不够高级

本文看自前端常用的库和实用技术之JavaScript按值传递和按引用传递

巴特西

前端常用的库和实用技术之JavaScript高级技巧

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