Interesting - questions about undefined

about undefined This thing , Many of them are used in ordinary development , But we really understand ?

today , Strange knowledge points appear
as everyone knows ,undefined It's undefined , This is often written in projects :

// Commonly used in initialization 
let str = undefined

No problem ! But the great God in the big factory ( Deep well ice ) Will study some magical problems ! Formulate some specifications , To define undefined It has to be written like this .

// It is used like this in big factories 
var str=void 0

This is the result

effect : This is to eliminate some hidden dangers
for example :

// If we write like this, there will be a problem 
function log(){
    
var undefined=1// because undefined Not a keyword , It can be defined as 
var a=undefined// Declare one more variable a, Its value is undefined
console.log(a)// What to print ???
}

there undefined Not in global variables undefined, But this declared variable undefined, So there will be problems with such an assignment , Print the 1.

summary : Although it could be js At the beginning of belt design , The legacy of the bug, But we still need to be meticulous and rigorous in our business .