报错消息
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)
错误格式
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。
例如
无效例子(也就是下面这么做会导致报这种错)
只读属性不能直接创建, 但我们可以通过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
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}; // 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; //
参见
Object.defineProperty()
Object.freeze()
- "Which animals have three lungs?" on answers.com
- Klingons (another answer to that query)