in、Object.keys

Array.prototype.getLength = function() {
    return this.length;
};
var arr = ['a', 'b', 'c'];
arr.name = 'June';
Object.defineProperty(arr, 'age', {
    enumerable: true,
    value: 17,
    writable: true,
    configurable: true
});
for(var i in arr) {
    console.log(i); 
}

const person = {
    name: 'mark',
    sex: 'male'
};
Object.prototype.age = 20;
for (let i in person) {
    console.log(i);
}
// name, sex, age
for (let j in person) {
    if (person.hasOwnProperty(j)) {
        console.log(j);
    }
}
// name, sex
// 拓展，获取对象自身所有属性
console.log(Object.getOwnPropertyNames(person))
// [name, sex]

function () {
    this.name = 'June';
}
Person.prototype.getName = function() {
    return this.name;
}
var person = new Person();
Object.defineProperty(person, 'age', {
    enumerable: true,
    value: 17,
    writable: true,
    configurable: true
});
console.log(Object.keys(person));
大专栏  in、Object.keysmment">// ['name', 'age']

// 1. 不会遍历到对象属性及其原型属性
Array.prototype.getLength = function() {
    return this.length;
};
var arr = ['a', 'b', 'c'];
arr.name = 'June';
Object.defineProperty(arr, 'age', {
    enumerable: true,
    value: 17,
    writable: true,
    configurable: true
});
for(let i of arr) {
    console.log(i); 
    // a,b,c
}

// 2. 如果要遍历对象，可与 Object.keys 配合
var person = {
    name: 'June',
    age: 17,
    city: 'guangzhou'
}
for(var key of Object.keys(person)) {
    console.log(person[key]); 
    // June, 17, guangzhou
}

//拓展
Object.entries()会返回多个由对象属性Key和Value组成的数组
const arr = {
    name: 'mike',
    age: 18
};
console.log(Object.entries(arr));
// [['name', 'mike'], ['age', '18']]

// 3. 配合 entries 输出数组索引和值/对象的键值
var arr = ['a', 'b', 'c'];
for(let [index, value] of Object.entries(arr)) {
    console.log(index, ':', value);
    // 0:a, 1:b, 2:c
}
var obj = {name: 'June', age: 17, city: 'guangzhou'};
for(let [key, value] of Object.entries(obj)) {
    console.log(key, ':', value);
    // name:June,age:17,city:guangzhou
}