JavaScript for in循环，for of循环

var person={fname:"Bill",lname:"Gates",age:56}; 
var txt = "";

// x 为属性名 
for (x in person) {
    txt = txt + person[x];
}
console.log(txt);

let iterable = [10, 20, 30];

for (let value of iterable) {
  console.log(value);
}
// 10
// 20
// 30

let iterable = [10, 20, 30];

for (const value of iterable) {
  console.log(value);
}
// 10
// 20
// 30

let iterable = new Map([["a", 1], ["b", 2], ["c", 3]]);

for (let [key, value] of iterable) {
  console.log(value);
}
// 1
// 2
// 3

for (let entry of iterable) {
  console.log(entry);
}
// [a, 1]
// [b, 2]
// [c, 3]

let iterable = new Set([1, 1, 2, 2, 3, 3]);

for (let value of iterable) {
  console.log(value);
}
// 1
// 2
// 3

// Note: This will only work in platforms that have
// implemented NodeList.prototype[Symbol.iterator]
let articleParagraphs = document.querySelectorAll("article > p");

for (let paragraph of articleParagraphs) {
  paragraph.classList.add("read");
}

var person={fname:"Bill",lname:"Gates",age:56}; 

// for–of循环并不能直接使用在普通的对象上，但如果我们按对象所拥有的属性进行循环，可使用内置的Object.keys()方法：
for (var key of Object.keys(person)) {
  console.log(key + ": " + person[key]);
}

// fname: Bill
// lname: Gates
// age: 56

<button onclick="myFunction()">点击这里</button>
<p id="demo"></p>
<script>
function myFunction(){
    var array = new Array();
    var x;
    var txt=""
    array[0] = 1;
    array[3] = 2;
    array[4] = 3;
    array[10] = 4;
    for( x in array ){
        alert(array[x]);     // 依次显示出 1 2 3 4
    } 
    alert(array.length);    // 结果是11
    for( var i=0 ; i<4 ; i++){
        alert(array[i]);     // 依次显示出 1 undefined undefined 2 
    }
    document.getElementById("demo").innerHTML = txt;
}
</script>