Knowledge points of a JavaScript variable declaration

Knowledge points of a JavaScript variable declaration_Basic knowledge

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Release： 2016-05-16 17:18:44

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After lunch last Thursday, the leader sent us a JavaScript question to do. There are people in our team who work on the front-end, some people who work on the back-end, and some who work on mobile web, so everyone has a different understanding of the question. , and then discuss it in the QQ discussion group. Although the findings are very basic, I gained a lot through discussions and shared them. Of course, from the perspective of developers with development experience, these are the most basic things for learning JavaScript. Because I usually use jQuery or third-party JS components, I don’t pay enough attention to basic JavaScript learning. The title is as follows. The question is: What are the output results of the two alerts?

Copy code The code is as follows:

type="text/javascript ">
var a = 1;
var a;
alert(typeof a);

(function () {
b = '-----';
var b;
})();
alert( typeof b);

My answer is: 1. undefined 2. undefined. Then the leader asked us to carefully consider the answer to the question. My analysis of the topic:
1. Declare a and assign the value to 1, and then redeclare a, but there is no value assigned at this time, then the default value of the variable should be undefined.
2. The b variable is a local variable in the function, and the global variable b is output in alert, so it is undefiend.
I ran the code myself in Chrome, and the correct result of the code is 1.number 2.undefined. What is examined here is the concept of JavaScript variable declaration in advance.
We are looking at another example, such as the following:

Copy the code The code is as follows:

test();

function test(){
alert("Hello World!");
}

The program will not report an error, but the running result is: Hello World!. Principle: Before the computer starts executing the statement, it will first search for all function definitions and then save the relevant functions.
Question 1:
var a = 1;
var a;
Declaring variable a on line 2 is equivalent to declaring a at the top, and then the first sentence is redeclaring a, and then Assign a value of 1. So typeof a is number
Question 2:
b = '-----';
var b;
Analysis of the second question: b='-----', program First, it will check whether there is a declaration of variable b in the context. If so, it will directly assign the value to '-----'. But alert(typeof b); is outside the function and outputs the global variable b, all of which are undefined.
Please note: assignment to variables is not done in advance.