在javascript中为undefined赋值 [英] Assigning value to undefined in javascript

问题描述

我正在用JavaScript编写一些代码.当我不小心遇到这个问题时.

I was writing some code in javascript. When I accidentally came across this.

undefined = 'some value' //does not give any error

true = 'some value';   //gives error

null = 'some value';   //gives error

第一个语句有效，而另两个无效.据我所知，undefined，true和null都是可以分配给某个变量的值，因此所有这些都应该是无效的语句.

how is it that first statement is valid whereas the other two are invalid. From what I know both undefined, true and null are values you can assign to some variable, so all these should be invalid statements.

推荐答案

来自 MDN :

undefined是全局对象的属性；即它是一个变量 在全球范围内. undefined的初始值为原始值 undefined.

undefined is a property of the global object; i.e., it is a variable in global scope. The initial value of undefined is the primitive value undefined.

因此，您可以将值分配给undefined，这与保留关键字true和null不同.请注意，NaN也是这种情况，它也不是保留关键字，因此，您可以为其分配任何值.

Hence, you can assign the value to undefined unlike true and null which are reserved keywords. Note that this is the same case with NaN as well which is again not a reserved keyword and hence, you can assign any value to it.

只需添加更多内容，即使您为undefined分配了一个值也没关系，因为它是只读属性，因此不会对其进行写入.

Just to add more to this, it doesn't matter even if you are assigning a value to undefined, it will not write to it as it is a readonly property.

再次从MDN报价.

在现代浏览器(JavaScript 1.8.5/Firefox 4+)中，undefined是 根据ECMAScript 5的不可配置，不可写属性 规格.即使不是这种情况，也请避免覆盖它.

In modern browsers (JavaScript 1.8.5 / Firefox 4+), undefined is a non-configurable, non-writable property per the ECMAScript 5 specification. Even when this is not the case, avoid overriding it.

建议在文件的最顶部或函数内部声明"use strict"，以在JavaScript中使用严格模式以避免此类情况.使用类似的东西

Prefer using strict-mode in your JavaScript by declaring "use strict" at the very top of the file or inside a function to avoid such things. Using something like

"use strict";
undefined = 'test'; //will raise an error, refer to [1]

^[1] VM1082:2未捕获的TypeError:无法分配为读取 对象#"的唯一属性未定义"

^[1] VM1082:2 Uncaught TypeError: Cannot assign to read only property 'undefined' of object '#'

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