JS function promotion and variable promotion

Problem background: In writing a very simple popup interaction, defined a global variable and a method, this method is always not adjusted to this variable, get expert guidance, deliberately summed up this knowledge point;

First, variable promotion

Before ES6, JavaScript had no block-level scope (a pair of curly braces {} is a block-level scope), with only global scope and function scope. The variable elevation is about to elevate the variable declaration to the beginning of the scope where it is located.

// undefined var global = ' global '//  globalfunction//  Undefinedvar a = ' aaa '//  AAA}fn ();　　

The reason is that the above printing results, is due to JS variable promotion, in fact, the above code is executed according to the following:

var // variable promotion, global scope, at this time only declared, and not assigned // undefined // The value is now assigned // Print out Global function　　fn () {  var//  variable promotion, function  scope = ' aaa '; Console.log (a);} fn ();

Second, function promotion

There are two ways to create functions in JS: function declarations and function literals. function promotion only exists for function declarations! Such as:

// function F1 () {}    // undefined   function F1 () {} var function () {}

Only so will have the above print result, is because the function promotion in JS causes the code to actually execute according to the following:

function // function Promotion, the entire code block is promoted to the beginning of the file <br> Console.log (   F1); Console.log (F2);    var function () {}

Three, the principle of summary

The function promotion is higher than the variable promotion priority! Lexical analysis Lexical analysis method:

JS Run before a similar compilation process is lexical analysis, lexical analysis is mainly three steps:

• Analysis parameters
• Declaration of the Re-analysis variable
• Analysis Function Description

The steps are as follows:

• The function generates an active object (active objects) at run-moment, called AO
• Analysis parameters

1. The function receives the form parameter, adds to the Ao property, and this time the value is undefine, for example Ao.age=undefine
2. Receives an argument, adds an attribute to the AO, overwrites the previous undefine

• Analyze variable declarations, such as Var age, or Var age=23;

1. If the AO has no age attribute in the previous analysis parameter, the AO attribute is added as Undefine, which is ao.age=undefine
2. If the AO above already has the age attribute, no modification is made

• The declaration of the parse function, if there is a functional age () {}

Assign a function to Ao.age, overwriting the value of the previous analysis

Write By:tuantuan

JS function promotion and variable promotion