function Expressions-mimic block-level scopes

Mimic block-level scopes

There is no concept of block-level scope in JavaScript, which means that the variables defined in the block statement are actually created in the containing function rather than in the statement.

1     function Outputnumbers (count) {2          for (var i = 0; i < count; i++) {3            console.log (i); 4         }5        console.log (i); 6     }

The local variable I defined in the for loop can then be accessed for out-of-loop, because the variable i is defined in the active object of outputnumbers (), so you can access it anywhere within the function, starting with its definition.

1     function Outputnumbers (count) {2          for (var i = 0; i < count; i++) {3            console.log (i); 4         }5         var  i; 6         Console.log (i); 7     }

In this case, he will only turn a blind eye to subsequent statements. Anonymous functions can be used to mimic block-level scopes and avoid this problem.

1     (function() {2         /// This is block-level scope 3     });

The above code defines and immediately invokes an anonymous function. The function declaration is contained in a stack of parentheses, which means that it is actually a function expression. The other pair of parentheses immediately after it is called.

Attention:

1     function () {2         // This is a block level scope . 3     // Error

The function keyword represents the beginning of a functional declaration, and the function declaration cannot be followed by parentheses. However, the function expression can be followed by parentheses. To convert a function declaration into a function expression.

You can use a private scope wherever you need some variables temporarily

1     function Outputnumbers (count) {2         (function() {3for              (var i = 0; i < count; i++) {  4                console.log (i); 5             }6        }) (); 7         // i am not defined 8     }

This technique is often used outside the function in the global scope to limit the addition of too many variables and functions to the global scope

1     (function() {2         varnew  Date (); 3         if (Now.getmonth () = = 0 && now.getdate () = = 1) {4             alert ("Happy New Year"); 5         }6     }) ();

Where now is the local variable in the anonymous function, and we don't have to create it in the global scope.

function Expressions-mimic block-level scopes