JavaScript的闭包

当内部函数被返回到外部并保存时，一定会产生闭包，闭包会产生，原来的作用域不能释放，过渡的闭包可能会导致内存泄漏，或加载过慢。

// 闭包做数据缓存
        function test() {
            var n = 100;
            function add() {
                n++
                console.log(n);
            }
            function reduce() {
                n--;
                console.log(n);
            }
            return [add, reduce]
        }
        var arr = test()
        arr[0]()//101
        arr[1]()//100
        arr[1]()//99
        arr[1]()//98
        arr[1]()//97
        arr[1]()//96

增删改查

   function breadMgr(num) {
            // var breadNum = num || 10
            var breadNum = arguments[0] || 10

            function storage() {
                breadNum += 10
                console.log(breadNum);
            }
            function sale() {
                breadNum--;
                console.log(breadNum);
            }
            return [storage, sale]
        }
        var breadMgr = breadMgr(50)
        breadMgr[0]()//60
        breadMgr[1]()//59

        function rechang(name) {
            var opt = {
                showThing: function () {
                    console.log("hello " + name);
                }
            }
            return opt
        }
        var things = rechang("mary")
        things.showThing()//hello mary

        function test() {
            for (var i = 0; i < 10; i++) {
                (function () {
                    document.writeln(i + ' ')
                })()
            }
        }
        test()//0 1 2 3 4 5 6 7 8 9

        function test() {
            for (var i = 0; i < 10; i++) {
                console.log(i);
            }
        }
        test()//0 1 2 3 4 5 6 7 8 9

        var a = []
        for (var i = 0; i < 10; i++) {
            a[i] = function () {
                console.log(i);
            }
        }
        a[5]()//10


        var b = []
        for (let i = 0; i < 10; i++) {
            b[i] = function () {
                console.log(i);
            }
        }
        b[5]()//5

形成闭包

    <script>
        // 把 add 方法返回出去赋值给了 adds 变量，然后adds变量成了一个新的函数，
        // 这个函数其实就是test里面那个add方法，里面的add方法拽着test的AO,
        // test的AO有num,所以可以任意修改
        function test() {
            var num = 0
            function add() {
                console.log(++num);
            }
            return add
        }
        var adds = test()
        adds()//1
    </script>

闭包缓存数据1

        function myClass() {
            var student = []
            var operations = {
                join: function (name) {
                    student.push(name)
                    console.log(student);
                    document.writeln(student + `<br>`)

                },
                leave: function (name) {
                    for (var i = 0; i < student.length; i++) {
                        var item = student[i]  //优化for循环性能的方法
                        if (item === name) {
                            student.splice(i, 1)
                        }
                    }
                    console.log(student + "123");
                    document.writeln(student + `<br>`)
                }
            }
            return operations
        }
        var opt = myClass()
        opt.join('张三')
        opt.join('李四')
        opt.join('王五')
        opt.leave('李四')

闭包缓存数据2

        function myClass() {
            var student = []
            var operations = {
                join: function (name) {
                    student.push(name)
                    console.log(student);
                    document.writeln(student + `<br>`)

                },
                leave: function (name) {
                    // for (var i = 0; i < student.length; i++) {
                    //     var item = student[i]  //优化for循环性能的方法
                    //     if (item === name) {
                    //         student.splice(i, 1)
                    //     }
                    // }
                    var idx = student.indexOf(name)
                    if (idx !== -1) {
                        student.splice(idx, 1)
                    }
                    console.log(student + "123");
                    document.writeln(student + `<br>`)
                }
            }
            return operations
        }
        var opt = myClass()
        opt.join('张三')
        opt.join('李四')
        opt.join('王五')
        opt.leave('李四')