In-depth understanding of JavaScript variable scope_javascript skills

Before learning the variable scope of JavaScript, we should clarify a few points:
a. The variable scope of JavaScript is based on its unique scope chain.
b. JavaScript does not have block-level scope.
c. The variables declared in the function are defined throughout the function.

1. JavaScript scope chain
First, take a look at the following code:

Copy code The code is as follows:

<script type="text/javascript" language="javascript">
var rain = 1;
function rainman(){
var man = 2;
function inner(){
var innerVar = 4;
alert(rain);
}
inner(); //Call inner function
}
rainman(); //Call the rainman function
</script>

Observe the code alert(rain);. JavaScript first checks whether the variable rain is defined in the inner function. If it is defined, the rain variable in the inner function is used. If the rain variable is not defined in the inner function, JavaScript will continue to check whether the rain variable is defined in the rainman function. In this code, the rain variable is not defined in the rainman function body, so the JavaScript engine will continue to look up (the global object) to see if rain is defined; in the global object, we have defined rain = 1, so the final result will pop up '1'.
Scope chain: When JavaScript needs to query a variable If the two objects are not defined, the search will continue, and so on.
The above code involves three scope chain objects, in order: inner, rainman, and window.

2. Within the function body, local variables have a higher priority than global variables with the same name.

Copy code The code is as follows:

<script type="text/javascript" language= "javascript">
var rain = 1; //Define the global variable rain
function check(){
var rain = 100; //Define the local variable rain
alert( rain ); / / 100 will pop up here
}
check();
alert( rain ); // 1 will pop up here
</script>

3. JavaScript There is no block-level scope.
This is also the more flexible part of JavaScript compared to other languages.
Look carefully at the code below, you will find that the scopes of variables i, j, and k are the same, and they are global in the entire rain function body.

Copy code The code is as follows:

<script type="text/javascript" language= "javascript">
function rainman(){
/**
* There are three local variables i j k in the rainman function body
*/
var i = 0;
if( 1 ){
var j = 0;
for( var k = 0 ; k < 3 ; k ){
alert( k ); //Pop up 0 1 2
}
alert( k ); //Pop up 3
}
alert( j ); //Pop up 0
}
</script>

4. The variables declared in the function are defined in the entire function.
First observe this code.

Copy code The code is as follows:

<script type="text/javascript" language= "javascript">
function rain(){
var x = 1;
function man(){
x = 100;
}
man(); //Call man
alert( x ); //100 will pop up here
}
rain(); //Call rain
</script>

The above code It shows that the variable x can be used throughout the rain function body and can be reassigned. Due to this rule, "unbelievable" results will be produced, observe the following code.
This is because the local variable x in the function rain is defined throughout the function body (var x= 'rain-man', declared), so the global variable x with the same name is hidden in the entire rain function body. The reason why 'undefined' pops up here is because when alert(x) is executed for the first time, the local variable x has not yet been initialized.

Copy code The code is as follows:

<script type="text/javascript" language= "javascript">
var x = 1;
function rain(){
alert( x ); //pop up 'undefined' instead of 1
var x = 'rain-man' ;
alert( x ); //pop up 'rain-man'
}
rain()
</script>

So the rain function above is equivalent to the function below.

Copy code The code is as follows:

function rain(){
var x;
alert( x );
x = 'rain-man';
alert( x );
}

5. Variables not defined using the var keyword are is a global variable.

Copy code The code is as follows:

<script type="text/javascript" language= "javascript">
function rain(){
x = 100; //Declare global variable x and assign value
}
rain();
alert( x ); / /Will pop up 100
</script>

This is also a common mistake for JavaScript newbies, leaving many global variables unintentionally.
6. Global variables are all attributes of the window object

Copy code The code is as follows:

<script type="text/javascript" language="javascript">
var x = 100;
alert( window.x);//pop-up 100
alert(x);
</script>

Equivalent to the following code

Copy code The code is as follows:

<script type="text/javascript" language="javascript">
window.x = 100;
alert( window.x );
alert(x )
</script>