js中的forEach用法注意点.html

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body></body>
<script type="text/javascript">
// forEach不改变原数组但是改变对象属性
let arr1 = [1,2,3,4];
arr1.forEach(item=>{
    item = 'a'
});
console.log(arr1);// [1, 2, 3, 4]  未改变原数组

let arr2 = [1,2,3,4];
arr2.forEach((item, index, arr)=>{
    arr[index] = 'b'
});
console.log(arr2); // ["b", "b", "b", "b"] 改变了原数组


//数组里面的子元素是对象，那么是可以改变对应属性的
var arr3 = [{a:1}, {a:1}];
arr3.forEach(item=>{
    item = null;
});
console.log(arr3);//[{a: 1} ,{a: 1}] 未改变原数组

var arr4 = [{a:1}, {a:1}];
arr4.forEach(item=>{
    item.a = 666;
});
console.log(arr4); //[{a: 666} ,{a: 666}] 改变了原数组
    

</script>
</html>