"Ascension" of function scopes in JavaScript

variable elevation of function scope in JavaScript

When we declare a variable with the keyword var inside the function, the scope of the variable is limited to the current function.

Elevation: Within a scope, regardless of where a variable is declared with Var, the variable belongs to the current entire scope and can be accessed anywhere in the current scope. In JavaScript, this behavior/phenomenon is called "Ascension", where a variable is declared with Var at any location in a scope, and theJavaScript engine "moves" the variable declared with Var to the beginning of the current scope.

When it comes to the "ascension" of JavaScript , I have to think of the time when I studied C in college, the old C language compiler, the C language, variables can only be declared and defined at the beginning of the function, the variables to be used in the function must be declared at the beginning of the definition , the declaration definition cannot have any other non-declaration-defined statements before it. Since the C + + language has broken this limit, the C new compiler does not have this hard limit.

It is not a good idea to rely on JavaScript as a "variable boost" mechanism. "Function Promotion" we generally use more.

Therefore, "function promotion" is safe to use, when the use of variables within a scope is located in the declaration of variables before the ground, and the resulting "variable promotion" phenomenon, we'd better not use.

Example of elevation phenomenon code:

var a = 2;foo (); function foo () {    a = 3;    Console.log (a);    var A;} Console.log (a);

Results:

 

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Elevation of function scope in JavaScript