javascript - 声明的 "undefined"值和未声明的 "undefined"值有什么区别？

Question

谁能解释一下第一个例子和第二个例子有什么区别？

声明的“未定义”值和未声明的“未定义”值有什么区别？

代码

var arr = new Array(4);
arr[0] = "a";
arr[1] = "b";
arr[2] = undefined; //insert undefined here!
arr[3] = "d";
console.log("arr[2] is " + arr[2]); //yes, it is undefined!

arr.forEach(function(value, index) {
    console.log(index + ":" + value);
})

console.log("====================")

var arr = new Array(4);
arr[0] = "a";
arr[1] = "b";
//I don't insert undefined to arr[2] in this case.
arr[3] = "d";
console.log("arr[2] is " + arr[2]); //yes, it is undefined!

arr.forEach(function(value, index) {
    console.log(index + ":" + value);
})

日志

arr[2] is undefined
0:a
1:b
2:undefined
3:d
====================
arr[2] is undefined
0:a
1:b
3:d

附加示例

var arr = new Array(4);
arr[0] = "a";
arr[1] = "b";
arr[2] = undefined; //insert undefined here!
arr[3] = "d";
console.log("arr[2] is " + arr[2]); //yes, it is undefined!

var i = 0;
var max = arr.length;
for(i; i < max; i++) {
console.log(i + ":" + arr[i]);
}

console.log("====================")

var arr = new Array(4);
arr[0] = "a";
arr[1] = "b";
//I don't insert undefined to arr[2] in this case.
arr[3] = "d";
console.log("arr[2] is " + arr[2]); //yes, it is undefined!

var i = 0;
var max = arr.length;
for(i; i < max; i++) {
console.log(i + ":" + arr[i]);
}   

日志

arr[2] is undefined
0:a
1:b
2:undefined
3:d
====================
arr[2] is undefined
0:a
1:b
2:undefined
3:d

Answer 1

Could anyone please explain what's the difference between the first example and the second example, please?

在第一个例子中:

var arr = new Array(4);

创建一个长度为 4 的数组，它没有元素。

然后使用直接赋值将一个值分配给索引 0 到 3:

arr[0] = "a";
arr[1] = "b";
arr[2] = undefined; //insert undefined here!
arr[3] = "d";

它创建属性 0 到 3。undefined 被分配给 arr[2]。 forEach 迭代存在的元素，因此您会看到所有 4 个元素的结果，每个元素都有一个值。

在第二个例子中，你没有给arr[2]赋值。当访问一个对象的不存在的属性时(注意数组是对象)，undefined 值被返回，例如

var obj = {};
console.log(obj.foo) // undefined

当使用 forEach 遍历属性时，不访问不存在的属性，因此 arr[2] 没有输出。这与 for 循环相反，后者通常用于访问从 0 到 length - 1 的所有属性，因此返回该范围内所有属性的值，无论它们是否存在还是不是。