Home > Web Front-end > JS Tutorial > body text

Variable declaration in JavaScript comes before assignment

高洛峰
Release: 2016-11-26 09:18:43
Original
1161 people have browsed it

As follows
var a = 3;
There are actually two steps:
1 Initialize a to undefined
2 Assign a to value 3

So there will be some "unbelievable" phenomena, that is, variables in JS can be used first and then declared. This is not allowed in Java.
System.out.println(a);
int a = 1;
Compilation fails. But JS can, as follows
alert(a);
var a;
Although it is undefined, no error will be reported. It means that a is indeed declared and is undefined.

If it is just "alert(a)" without "var a", the JS engine will report an error.
alert(a);
FF is as follows

Variable declaration in JavaScript comes before assignment

Although it can be used first and then declared, this will cause the effect of the assignment to be lost. As follows
alert(a);
var a = 1;
This time the output is still undefined instead of 1.

Another example,
alert('a' in window); // true
var a;
Although the code is formally written after alert, the engine still automatically processes the var declaration first. The final output is true.

After understanding this, it is not difficult to understand the results of the following code

if (!("a" in window)) {
var a = 1;
}
alert(a);


Related labels:
source:php.cn
Statement of this Website
The content of this article is voluntarily contributed by netizens, and the copyright belongs to the original author. This site does not assume corresponding legal responsibility. If you find any content suspected of plagiarism or infringement, please contact admin@php.cn
Popular Tutorials
More>
Latest Downloads
More>
Web Effects
Website Source Code
Website Materials
Front End Template