Undefined instance analysis in js

When we develop, we often encounter undefined situations. This article mainly shares with you a summary of undefined problems in js, hoping to help everyone.

 //1.变量声明了但是没赋值 解析器会给一个默认值 就是undefined
    var a;
    console.log(a);
    //2.数组中 某一项没有值 值是undefined
    var arr = [1, 2, 3];
    console.log(arr[10]);
    //3.形参接收不到值 接收到的就是undefined
    function fn(a, b) {
        console.log(b);
    }
    fn(1);
    //4.函数没有返回值 相当于返回了undefined
    console.log(fn(1, 2));
    //5.对象没有这个属性 非要获取这个属性的值 这个属性的值也是undefined
    var obj = {
        name: "zs",
        age: 18
    };
    console.log(obj.sex);
    //undefined都是解析器给的默认值
    //null一般都是程序员主动赋予的值
    var arr = [];
    arr.push();

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