var arr = ["first","second","third","fourth",100,200,300];
for(var i = 0; i < arr.length;i++){
console.log(arr[i]);
}
//输出：
first
second
third
fourth
100
200
300
var arr = ["first","second",‘third‘ ,"fourth",100,200,300];
for(var i in arr){
console.log(arr[i] +‘/‘ + i);
}
//输出结果为：
first/0
second/1
third/2
fourth/3
100/4
200/5
300/6
var arr = ["first","second",‘third‘ ,"fourth",1,2,3];
for(var item of arr){
console.log(item);
}
//输出结果：
first
second
third
fourth
1
2
3
虽然for… in 、 for… of 都能够变历数组，但是两者还是有很大区别的，先说结论：
两者的主要区别在于他们的迭代方式
var arr = ["first","second","third","fourth",100,200,300];
//element 表示arr的单元项，index 表示arr单元项对应的索引值
arr.forEach(function(element,index){
console.log(element + ‘/‘ + index);
})
//输出结果：
first/0
second/1
third/2
fourth/3
100/4
200/5
300/6
注意：未赋值的值是不会在foreach循环迭代的，但是手动赋值为undefined的元素是会被列出的
var arr = ["first","second",‘third‘ ,"fourth"];
var arr2 = arr.map(function(item){
return item.toUpperCase();
})
console.log(arr2);
//输出：
[FIRST,SECOND,THIRD, FOURTH]
var arr = ["first","second",‘third‘ ,"fourth",100,200,300];
var arr3 = arr.filter(function(item){
if(typeof item == ‘number‘){
return item;
}
})
console.log(arr3);
//输出结果：
[100,200,300]
every()与filter()的区别是：后者会返回所有符合过滤条件的元素；前者会判断是不是数组中的所有元素都符合条件，并且返回的是布尔值
var arr = ["first","second",‘third‘ ,"fourth",100,200,300];
var bol = arr.every(function(element){
if(typeof element == ‘string‘){
return element;
}
})
console.log(bol); //false
every()与some()的区别是：前者要求所有元素都符合条件才返回true,后者要求只要有符合条件的就返回true
var arr = ["first","second",‘third‘ ,"fourth",100,200,300];
var bol = arr.some(function(element){
if(typeof element == ‘string‘){
return element;
}
})
console.log(bol); //true
原文：https://www.cnblogs.com/zhuyan0000/p/10847429.html