JavaScript variable scope

The variable scope is the region in the program that defines this variable.

First, paste a piece of code. If you are not confused about the output of the code, you do not need to read it below.

/* Code 1 */
Var scope = "global ";
Function checkScope (){
Var scope = "local ";
Function childCheck (){
Var scope = "childLocal ";
Document. write (scope );
}
Function childUndefined (){
Document. write (scope );
Var scope;
}
Function childOverride (){
Scope = "childOverride ";
Document. write (scope );
}
Document. write (scope); // output "local"
ChildCheck (); // output "childLocal"
ChildUndefined (); // output "undefined"
ChildOverride (); // output "childOverride"
Document. write (scope); // output "childOverride"
}
CheckScope (); // output "local childLocal undefinedchildOverride childOverride"
Document. write (scope); // output "global"
Global scope and local scope
The scope of global variables is global and is defined everywhere in Javascript. The variables declared in the function are local variables whose scope is local, it is defined only within the function body. The following output readers should not be surprised.

/* Code 2 */
Var scope = "global ";
Function checkScope (){
Var scope = "local ";
Document. write (scope );
}
CheckScope (); // output "local"
Document. write (scope); // output "global"

You do not need to use the var statement when using a variable in the global variable scope. However, when declaring a local variable, you must use the var statement. Otherwise, it is considered as a reference to the global variable. See the following code:

/* Code 3 */
Var scope = "global ";
Function checkScope (){
Scope = "local ";
Document. write (scope );
}
CheckScope (); // output "local"
Document. write (scope); // output "local"

No block Scope
Javascript has no block-level scope, and the variables declared in the function are defined throughout the function. The following code may surprise unfamiliar readers:

/* Code 4 */
Var scope = "global ";
Function checkScope (){
Document. write (scope); // statement 4.1
Var scope = "local"; // statement 4.2
Document. write (scope );
}
CheckScope (); // output "undefinedlocal"

Because the variables declared in statement 4.1 (var scope = "local";) are valid throughout the scope of the checkScope function, in statement 4.2 (document. when write (scope);) is executed, scope references local variables. At this time, the local variable scope has not been defined, so the output is "undefined ". Therefore, a good programming habit is to put all the variable declarations together at the beginning of the function.

After learning about the above content, the reader should not be confused when looking at Code 1.

Object Property Variables

Attribute variables of an object are easy to understand. Readers should not be confused when looking at the code below the samples. /* Code 5 */
Var scope = "global ";
Var obj = new Object ();
Obj. scope = "object ";
Obj. checkScope = function (){
Var scope = "loacl ";
Document. write (scope); // output "loacl"
Document. write (this. scope); // output "object"
Document. write (window. scope); // output "global"
}
Obj. checkScope (); // output "loacl object global"