Why undefined can be covered by with while null can't?

user_7743089 0
Viewed 49

Why can undefined be covered by with while null can't? And also, let can be covered while var can't (it will throw a error).

So what is the rule that a variable can be redefined?

enter image description here

var obj = {null:1,undefined:2};
with(obj) console.log(null,undefined);

null undefined javascript
1 Answers

Because undefined is a predefined global identifier, but null is a literal. Because var is a keyword, but let isn't (in loose mode; it is in strict mode). (In loose mode, it acts like a keyword or identifier depending on where it appears.)

I don't recall why undefined is a predefined global rather than being a keyword. The reason let is only a keyword in strict mode is that it was only added in 2015, 20 years after the language was first defined.

user_1572470
Related