JavaScript variable declaration in advance

The topic is as follows: Please write out the following code 2 times alert output what results separately?

<script type= "Text/javascript" >var a = 1; var A;alert (typeof a); (function () {b = ' Hello world '; var b;}) (); Alert (typeof B); </script>

My answer is: 1.undefined 2.undefined. Then leader let's think about the answer to the question. My analysis of the topic: 1. Declare a and assign a value of 1, and then re-declare a, but there is no assignment at this time, the variable is declared, but not assigned, so it is undefined. 2. The b variable in the first row is a global variable inside the function, and then a local variable is declared, but no value is assigned. The alert output is a global variable B, so it is also undefiend.

I ran the code myself in Chrome, and the correct result was 1.number 2.undefined. This is a review of JavaScript's variable declaration advance concept.

Test (); function test () {     alert ("Hello world!");}

The result of the operation is: Hello world!. Principle: Before the computer starts executing the statement, it finds all the function definitions and then saves the associated function.

Analysis results:

Question 1th:var a = 1; var A;

The 2nd line declares the variable a, which is equivalent to declaring a at the top, and then the first sentence is to re-declare a and assign a value of 1. So typeof A is number

Question 2nd: b = ' Hello world '; var b;

Second question: b= ' Hello World ', the program will first look inside the function to see if there is a declaration of variable B, if any, directly assigned to ' Hello World '. But alert ( typeof b); is outside the function, the output of global variable B, all is undefined.

Then look at the following code snippet:

<script type= "Text/javascript" >    name = "AAA";    function test () {        alert (typeof name);        var name = "BBB";        Alert (typeof name);    }    Test ();</script>

Please write the results.

The analysis can be written in the following code snippet:

myname = "AAA"; function test () {        alert (typeof myname);//Find out if there is a myname declaration within the function, there is a declaration, but no value assigned. So typeof myname for undefined  var myname=123;//assignment Action   alert (typeof myname);//number} test ();

But what is the result of the following code snippet?

<script type= "Text/javascript" >    alert (typeof name);    var name = "Hello World";    Alert (typeof name);</script>

The result of the program operation is: String,string. What is the reason?

Note: The results have been obtained through a joint discussion with the Friends of the park. As for the result of the last string and string, it is true that the Chrome browser defaults to name Name= "", so the result of the output is string. Here we are reminded that in the front-end development process, we are often concerned about CSS and HTML in the various browser compatibility, but sometimes JavaScript in the browser also has compatibility issues.

Reprinted from: Liminjun88 (including link http://www.cnblogs.com/liminjun88/)

JavaScript variable declaration in advance