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Showing content from https://developer.mozilla.org/zh-CN/docs/Web/JavaScript/Reference/Errors/Read-only below:

TypeError: "x" is read-only - JavaScript

TypeError: "x" is read-only (Firefox) //格式错误："x"只读。(x 可以代表任意值)
TypeError: 0 is read-only (Firefox)
TypeError: Cannot assign to read only property 'x' of #<Object> (Chrome)
//格式错误：对象的 x 属性是只读的不能设置（chrome）
TypeError: Cannot assign to read only property '0' of [object Array] (Chrome)

全局变量或对象属性被设置为只读 (专业点讲，就是这条数据属性禁止写入.)

这条错误值发生在strict mode code(俗称严格模式). 正常情况下，是没有报错的。

只读属性不能直接创建，但我们可以通过Object.defineProperty() 或 Object.freeze()设置。

"use strict";
var obj = Object.freeze({name: "Elsa", score: 157});
obj.score = 0;  // TypeError

"use strict";
Object.defineProperty(this, "LUNG_COUNT", {value: 2, writable: false});
LUNG_COUNT = 3;  // TypeError

"use strict";
var frozenArray = Object.freeze([0, 1, 2]);
frozenArray[0]++;  // TypeError

还有几个 JavaScript 内置属性。如果你尝试修改一个常量。

"use strict";
Math.PI = 4;  // TypeError

傻了吧，报错了

全局变量 undefined 也是只读的, 所以你不能忽视臭名昭著的"undefined is not a function"错误：

"use strict";
undefined = function () {}; // TypeError: "undefined" is read-only

"use strict";
var obj = Object.freeze({ name: "Score", points: 157 });
obj = { name: obj.name, points: 0 }; // 用一个新对象替换原来的对象 (其实就是更改了对象的指针)

("use strict");
var LUNG_COUNT = 2; //
LUNG_COUNT = 3; //

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