JavaScript study notes: Variables and their scope_Basic knowledge

1. Variables

ECMAscript variables are loose variables. The so-called loose variables mean that the variable name can save any type of data. Each variable is just a placeholder for saving a value.

Definition: var firstDemo;

2. Scope of variables

2.1 Basic Concepts

Use var to define variables: define local variables in the scope of the variable. This method of defining variables is also called an explicit declaration.

If you don’t understand this, you can take a look at the simple and crude example below:

Copy code The code is as follows:

test();
function test(){
var firstDemo="hello";//Define local variables
alert(firstDemo);//hello
}
test();
function test(){
var firstDemo="hello"; //Define local variable firstDemo
} 
alert(firstDemo);//Error report, firstDemo is not define

As can be seen from the above two examples, if you use var to define a variable in a function, the variable will be destroyed after the function exits.

Omit var to define a variable: As long as the function that defines the variable is called once, the variable can be accessed in the global scope. This method of defining variables is also called implicit declaration

Copy code The code is as follows:

tips: Explicitly declared variables are already compiled into the calling object during precompilation (for example, var t=1; execute var t during precompilation; execute t=1 during interpretation), which is different from implicit Declared variables are not defined as global variables until they are interpreted.

Clearing the scope of variables can help us think about how to declare variables reasonably, which not only reduces unnecessary memory overhead, but also largely avoids the trouble caused by repeated definitions of variables overwriting previously defined variables. .

2.2 Scope Analysis

Copy code The code is as follows: