Errors: Read-only

信息

TypeError: "x" is read-only (Firefox)
TypeError: 0 is read-only (Firefox)
TypeError: Cannot assign to read only property 'x' of #<Object> (Chrome)
TypeError: Cannot assign to read only property '0' of [object Array] (Chrome)

错误类型

TypeError

哪里出错了?

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

示例

无效的情况

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

JavaScript中还内置了一些只读属性。尝试重新定义一个数学常数。

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

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

下面这样都是有效，不报错的

'use strict';
var obj = Object.freeze({name: 'Score', points: 157});
obj = {name: obj.name, points: 0};   // replacing it with a new object works

'use strict';
var LUNG_COUNT = 2;  // a `var` works, because it's not read-only
LUNG_COUNT = 3;  // ok (anatomically unlikely, though)