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What is the usage of in in javascript

醉折花枝作酒筹Original: 2021-07-16 13:30:455030browse

The usage of in in javascript is: 1. Determine whether the attribute exists in the object. If it exists, the return value is true. If it does not exist, it is false; 2. Determine the array. For the array, the index The number is an attribute, and you can use an array index or a string.

The operating environment of this tutorial: windows7 system, javascript version 1.8.5, Dell G3 computer.

<!DOCTYPE html>
<html>
<head>
    <meta charset="UTF-8">
    <title>Document</title>
</head>
<body>
    
    <script type="text/javascript">
        // for-in 一般用来遍历对象, 也可以用来遍历数组
        // var obj = {
        //     name: "xiaoming",
        //     age: 8
        // };
        // for(var k in obj) {
        //     // 此处使用 点运算符，不能获取到相应的值：
        //     // . 的作用：访问对象的属性
        //     console.log("键为：" + k + ", 值为：" + obj[k]);
        // }

        // var arr = [1, 3, 5];
        // for(var k in arr) {
        //     console.log("键为：" + k + ", 值为：" + arr[k]);
        // }

        // in 运算符作用：
        // 就是判断 属性是否存在于对象中，如果存在，返回值为：true
        // 如果不存在，则为：false
        // 语法：属性 in 对象
        // var obj = {
        //     name1: "jack",
        //     age: 9,
        //     abc: undefined
        // };

        // 如果是name，要注意：window有name属性
        // console.log("name1" in obj); // true
        // console.log("age" in obj);
        // console.log("age123" in obj);

        // 如果是对象中存在的成员或者是原型中的成员，此时，返回的结果就是 true
        // console.log("toString" in obj);
        // console.log(obj.toString());
        // console.log("abc" in obj);


        // in运算符判断数组
        // 对于数组来说，索引号 就是属性
        var arr = [1];
        // console.log("1" in arr); // false
        // console.log("0" in arr); // true
        // console.log(0 in arr); // true

        // 访问数组的成员：
        // 可以使用 数组索引 也可以使用 字符串
        console.log(arr[0]);
        console.log(arr["0"]);


    </script>
</body>
</html>

[Recommended learning: javascript advanced tutorial]

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