前端（二十一）：label语句、with语句、JSON、生成器、解析赋值、历史状态管理、将页面可编辑

label语句

let data = {
    a: 'a', b: 'b', c: 'c', d: 'd', e: 'e' }

// break 跳出switch循环，但是for循环继续
// no a => b => no c => no d => no e
for (key in data) {
   
    switch (key) {
   
        case "b":
            console.log(key);
            break;
        default:
            console.log('no', key);
    }
}

// label语句 当满足条件后跳出for循环
// no a => b
outloop: for (key in data) {
   
    switch (key) {
   
        case "b":
            console.log(key);
            break outloop;
        default:
            console.log('no', key);
    }
}

width 将作用域设置到特定的对象中

a = 'AAA';
b = 'BBB';
let data = {
    a: 'aaa' }
with(data){
   
    console.log(a); // aaa
    console.log(b); // BBB
}

JSON stringify

var foo = {
    name: "Lee", age: 24, sex: '男' };
var jsonString = JSON.stringify(foo, (key, value) => {
   
    console.log(key, value); // name Lee --- age 24 --- sex 男
    if (typeof value === 'number') {
   
        return undefined
    }
    return value
});
console.log(jsonString); // {"name":"Lee","sex":"男"}

const students = [
    {
   
        name: 'akira',
        score: 100,
    }, 
    {
   
        name: 'John',
        score: 60,
    }, 
    {
   
        name: 'Jane',
        score: 90,
    }
];
let jsonStudents = JSON.stringify(students, (key, value) => {
   
    if (key === 'score') {
   
        if (value === 100) {
   
            return 'S';
        } else if (value >= 90) {
   
            return 'A';
        } else if (value >= 70) {
   
            return 'B';
        } else if (value >= 50) {
   
            return 'C';
        } else {
   
            return 'E';
        }
    }
    return value;
});
console.log(jsonStudents); // [{"name": "akira","score": "S"},{"name": "John","score": "C"},{"name": "Jane","score": "A"}]

let jsonStudentsNames = JSON.stringify(students, ['name']);
console.log(jsonStudentsNames); // [{"name":"akira"},{"name":"John"},{"name":"Jane"}]

JSON parse

let json = '[{"name": "akira","score": "S"},{"name": "John","score": "B"},{"name": "Jane","score": "A"}]';
let arr = JSON.parse(json, (key, value)=>{
   
    if(key === 'score'){
   
        switch(value){
   
            case 'S':
                return '优';
            case 'A':
                return '甲';
            case 'B':
                return '乙';
        }
    }
    return value;
})
console.log(arr); // [{name: "akira", score: "优"},{name: "John", score: "乙"},{name: "Jane", score: "甲"}]

生成器

function* addCount(){
   
    let count = 0;
    while (true) {
   
        yield count++
    }
}
let add = addCount();

console.log(add.next()); // {value: 0, done: false}
console.log(add.next()); // {value: 1, done: false}
console.log(add.next()); // {value: 2, done: false}

解析赋值

var [name, age] = ['Lee', 24];
console.log(name);
console.log(age);

交换

let a = 1;
let b = 2;
[a, b] = [b, a]
console.log(a); // 2
console.log(b); // 1

对象解析赋值

let obj = {
   
    name: 'Lee',
}

let {
   name} = obj; // => let {name: name} = obj; 
console.log(name); // Lee

let {
   name: a} = obj;
console.log(a); // Lee

let obj = {
   
    name: 'Lee',
    age: 24
}
let {
   name: _name, age: _age} = obj;
console.log(_name); // Lee
console.log(_age);  // 24

pushState popstate

window.onload = function () {
   
    pushHistory();
    setTimeout(function () {
    
        window.addEventListener('popstate', function (event) {
   
            let e = event || window.event;
            console.log(e);
        }, false);
    }, 100);
}
function pushHistory() {
   
    window.history.pushState({
    name: "Lee" }, "----", "home.html");
}

页面可编辑

document.designMode = 'on'