"Precompilation phase" and "execution phase" during JavaScript run

JavaScript is a weak type of language relative to other languages, and in other Java languages, the execution of a program requires a compile stage

There is a similar "precompilation phase" in JavaScript (JavaScript precompilation is scoped to a block of code, i.e., each code block encountered is precompiled > <script></script> execution ),

First, in the popular science of JavaScript in two ways of declaring, Var and function, the former declares a variable, the latter is a method of declaring

In precompilation, JavaScript makes two kinds of processing schemes for both declarations.

<script> var a = "1";　　Declare variable a   function  B () {//Declaration method B      alert ();　　}  varfunction() {    //Declare variable C alert (); }</script>

In the above code block, a, c is the variable assignment, B is the function declaration, when the above code is executed,

The first step is to go to the pre-compilation stage

Assigning A to a variable, C creates a memory space in memory and points to the variable name, and assigns a value of undefined

For function declarations, memory space is also created, but assigned objects assign values to function names

precompilation phase: (PS: Variables are declared and functions are declaredin the precompilation phase, regardless of the order in which the variables are declared and declared in the code)

<script> var a = undefined; 　　　　 var c = undefined;
　　　　

var B = function() {alert (); }

</script>

Implementation phase:

<script>= "1"function() {alert (); }</script>

Overall implementation steps:

<script> var a = undefined;　　　　　　　　 var c = undefined;　　　　 varfunction= "1"function() {alert (); }</script>

Topic:

<script> var a = "1";　　　　　　 function  B () {alert (a);　　　　 var a = "2"; } b (); </script>

PS: Variables and function declarations relative to the window environment, each scoped to the variables and functions under it

<script>      Functionhello () {        alert ("Hello");       }        Hello ();  　　</script>   <script>   Functionhello () {          alert ("Hello World");     }       Hello ();   </script>

<script>   

function Hello () {        alert ("Hello");       }        

function Hello () {          alert ("Hello World");     }       Hello ();  </script>

Blog: http://www.cnblogs.com/alex-web/

Note: The yy of the Little crazy paper

"Precompilation phase" and "execution phase" during JavaScript run