Summary of JS Pre-parsing problem

// Example 1alert (a)       //  undefind. alert (FN)      //  function whole block.  var a = 1; function fn () {     return  falss;};

Why? The first is undefind that the second is a functin function block. How is this going to work?

The JS parser is divided into two steps when running the JS code:

1. The first step is to move all 变量 , 函数 参数 forward, to the top of the current scope.
2. The second step is to interpret the code by line 从左到右 从上至下 .

//Example 1 explanationvarA//The a variable is advanced to the front of the entire scope before the current value is undefind. functionfn () {return false;}; //The FN function precedes the entire scope before the current value is the entire function block. alert (a); //Undefind. Alert (FN);//function the entire block of functions. A=1;//a assignment 1 now has value for a variable: 1functionfn () {return false;}; //The function has not changed but was preceded by the pre-scope. I remember saying something like this:                    //The function in that definition is not important to call on that.                     //(because it will be preceded by the current scope after the definition, in which scope the call takes effect)

Tips: The parsing engine performs the creation of all Var variables in block-level , giving an initial value of undefined.

alert ( a);           // a function block.  var a = 1; alert (a)          //  a assigns a value equal to 1.  function  A () {    returnfalse;}

//Example 3 explanationvarA//first, the current value of the advance variable is: Undefind. functionA () {return false;}; //Encounter function Advance function Note: The current function is the same as the above variable name, how to do?                         //don't worry. The JS parser has already done a function with the same name will replace the same name of the variable now only left a function block itself. alert (a); //because only a function is left above, the entire function block of the A function is now ejected. A = 1;//A = 1 expression is assigned so the A function is replaced with the variable value: 1. alert (a);//the value that pops up now is a value of 1 that has been changed;functionA () {//This function is useless now.     return false;};

Pre-parse with parameters: notice there's a scope problem here.

// Example 4 var a = 1; function Fn1 (a) {    alert (a);     = 2;} ; Fn1 ();                                 // Undefind. alert (a);                             // A value is: 1. 

//Example 4 explanationvarA//A variable advance current value: Undefind. (Global variables)functionFn1 (a) {alert (a); A= 2;} ;//FN1 function Block Advance value is: entire function                                      //The advance is all in advance and now begins to interpret the code line by row,                                      //wait, somebody's going to say the parameters? Hahaha don't worry about it yet. varA = 1;//A = 1 An assignment expression changes the value of a variable A to: 1. functionFn1 (a) {varA//parameters are parsed as variables to the top of the current function. alert (a);//a value in the current function is: undefined.A = 2;//A = 2 copy expression is useless}；//function blocks do not experience calls that are useless. FN1 ();//Now that we have a call to function fn1, let's go inside and see.                                       //after the call succeeds, the Undefind is bounced. alert (a);//This is still called global variable A with a value of 1. 

Summary of JS Pre-parsing problem