Block-level scope for JS

Today brings the summary of block-level scopes in JS you do not know to share:

1) The scope created with the From object is valid only in the with declaration, not the outer scope , and can access the properties of an existing object and add it to an existing object

Code Demo:

         var obj = {             A:1,             B:2,             C:3         };           with (obj) {             a=3;             b=4;             C=5;
             d=6;         }         Console.log (obj); // 3,4,5

2) The catch clause of the Try/catch creates a block-level scope in which the declared variable is valid only within the catch

Code Demo:

        Try {            undefined (); // perform an illegal operation to force an exception to be created         }        catch(err) {            console.log (err); // be able to perform normally         }        Console.log (err); // Referenceerror:err is not defined

3) Let usage: You can bind a variable to any scope in which it resides (usually {...} Internal

Code Demo:

          for (i = 1; i < 5; i++) {             console.log (i); // 1 2 3 4          }         console.log (i); // 5          for (Let j = 1; j < 5; J + +) {             console.log (j); // 1 2 3 4          }         Console.log (j); // referenceerror:j is not defined

Because let creates a block-level scope, external access to the Let declaration of the variable

4)const: can be used to create block scope variables; The value is fixed ( constant ), and any attempt to modify the value will cause an error

Code Demo:

        varFoo =true; if(foo) {varA = 2; Const B= 3;//block scope constants that are contained in the IFA = 3;//Normalb = 4;//ErrorConsole.log (b);//typeerror:invalid assignment to const ' B ' (this shows that its value is constant and cannot be changed after that)} console.log (a);//PConsole.log (b); //referenceerror! ( This exception can prove that the const created block scope)

Block-level scope for JS