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 :
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.
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]
这篇关于在javascript中为undefined赋值的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！