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Showing content from https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Operators/in below:

in - JavaScript | MDN

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如果指定的属性在指定的对象或其原型链中，则 in 运算符返回 true。

const car = { make: "Honda", model: "Accord", year: 1998 };

console.log("make" in car);
// Expected output: true

delete car.make;
if ("make" in car === false) {
  car.make = "Suzuki";
}

console.log(car.make);
// Expected output: "Suzuki"

prop

一个字符串类型或者 symbol 类型的属性名或者数组索引（非 symbol 类型将会强制转为字符串）。

objectName

检查它（或其原型链）是否包含具有指定名称的属性的对象。

下面的例子演示了一些 in 运算符的用法。

// 数组
var trees = new Array("redwood", "bay", "cedar", "oak", "maple");
0 in trees; // 返回 true
3 in trees; // 返回 true
6 in trees; // 返回 false
"bay" in trees; // 返回 false (必须使用索引号，而不是数组元素的值)

"length" in trees; // 返回 true (length 是一个数组属性)

Symbol.iterator in trees; // 返回 true (数组可迭代，只在 ES2015+ 上有效)

// 内置对象
"PI" in Math; // 返回 true

// 自定义对象
var mycar = { make: "Honda", model: "Accord", year: 1998 };
"make" in mycar; // 返回 true
"model" in mycar; // 返回 true

in右操作数必须是一个对象值。例如，你可以指定使用String构造函数创建的字符串，但不能指定字符串文字。

var color1 = new String("green");
"length" in color1; // 返回 true
var color2 = "coral";
"length" in color2; // 报错 (color2 不是对象)

如果你使用 delete 运算符删除了一个属性，则 in 运算符对所删除属性返回 false。

var mycar = { make: "Honda", model: "Accord", year: 1998 };
delete mycar.make;
"make" in mycar; // 返回 false

var trees = new Array("redwood", "bay", "cedar", "oak", "maple");
delete trees[3];
3 in trees; // 返回 false

如果你只是将一个属性的值赋值为undefined，而没有删除它，则 in 运算仍然会返回true。

var mycar = { make: "Honda", model: "Accord", year: 1998 };
mycar.make = undefined;
"make" in mycar; // 返回 true

var trees = new Array("redwood", "bay", "cedar", "oak", "maple");
trees[3] = undefined;
3 in trees; // 返回 true

如果一个属性是从原型链上继承来的，in 运算符也会返回 true。

"toString" in {}; // 返回 true

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