JavaScript Variable Declaration Promotion

    (function() {        var x=foo ();         var foo=function  foo () {            return ' Foobar '        };         return x;    }) ();

This code runs after the error: Uncaught Typeerror:foo is not a function

The reason is that the declaration of the variable foo is lifted, the subsequent assignment, and the function expression is not lifted, so when the code runs to Var x=foo (); When

Foo () is undefined.

Can put Var x=foo (); Put the variable foo before executing:

(function() {        var foo=function  foo () {            return ' Foobar '        };         var x=foo ();         return x;    }) ();

The function declares the entire promotion to the top of the scope, such as writing:

(function() {        var x=foo ();         function foo () {            return ' Foobar '        }        return  x;    }) ();

Let's look at an example:

    var foo = {N:1};    (function//Note here, pass in parameter foo         console.log (FOO.N);        FOO.N=3//global variable re-assignment        ,var foo={n:2//local variable foo is re-assigned        Console.log ( FOO.N);    }) (foo);    Console.log (FOO.N); //global variable is already N:3 

• First step: Pre-compile, VAR global variable foo, anonymous function functions, var local variable foo
• Second step: The code performs the calculation from top to bottom, from left to right:

1. Assign the global variable foo foo={n:1}; Note: This value is an object, which belongs to the reference type;
2. anonymous function passed in parameter foo={n:1} self-executing;
3. Console.log (foo); number 1;
4. Because of the Foo local variable, the foo variable is assigned a value of Foo={n:3}, and the parameter value of the reference type is changed, and the global foo variable is re-assigned the value Foo={n:3};
5. The local variable foo is re-assigned foo={n:2};
6. Console.log (foo); number 2;
7. global variable Foo={n:3}, so Console.log (foo); hit the number 3;

about the var x1=foo (); var X2=foo; There is no parenthesis difference

A function is an instance of a type called a function reference, so it is an object. The object is stored in memory, and the function name is a pointer to the object.

A function can be passed as a parameter to another function, or it can be a return value of a function, or it can be re-assigned.

In simple terms, X1 is the return value of function foo (), and X2 is the function foo () itself.

(function() {        var foo=function  foo () {            return ' Foobar '        };         var x=foo;         return x;    }) (); // function foo () {return ' Foobar '}    

JavaScript Variable Declaration Promotion