js数组遍历和对象遍历小结

数组的遍历

• for循环

for(var j = 0,len = arr.length; j < len; j++){
    console.log(arr[j]);
}

• forEach,性能比for还弱

arr.forEach(function(value,i){
    console.log('forEach遍历:'+i+'--'+value);
})

• map遍历

arr.map(function(value,index){
    console.log('map遍历:'+index+'--'+value);
});

• for-of遍历 是ES6新增功能

for( let i of arr){
    console.log(i);
}

forEach、map是ECMA5新增数组的方法，for-of是ES6新增方法,ie9以下的浏览器还不支持

对象遍历

• for in

for(var key in obj){
    console.log(key+": "+obj[key])
}

• forEach

obj.forEach((value,index)=>{
    console.log(value+","+index)
})

补充

• for-of

    // for-of遍历数组，不带索引，i即为数组元素
    for(let i of arrTmp){
        console.log(i)
    }
    //输出 "value1" "value2" "value3"

    // for-of遍历Map对象
    let iterable = new Map([["a", 1], ["b", 2], ["c", 3]]);
    for (let [key, value] of iterable) {
      console.log(value);
    }
    //输出 1 2 3

    // for-of遍历字符串
    let iterable = "china中国";
    for (let value of iterable) {
      console.log(value);
    }
    //输出 "c" "h" "i" "n" "a" "中" "国"

• Object.keys()返回键名数组

    //数组类型
    let arr = ["a", "b", "c"];
    console.log(Object.keys(arr));
    // (3) ['0', '1', '2']

    // 类数组对象
    let anObj = { 100: 'a', 2: 'b', 7: 'c' };
    console.log(Object.keys(anObj));
    // (3) ['2', '7', '100']

    //一般对象
    let xyz = {z: "zzz", x: "xxx", y: "yyy"};
    console.log(Object.keys(xyz));
    // (3) ["z", "x", "y"]