Learn JavaScript from the beginning (ii)--variables and their scopes

Original: Learn JavaScript from the beginning (ii)--variables and their scopes

One, variable

ECMAScript variables are loose variables, so-called loose variables, that is, variable names can hold any type of data, each variable is just a placeholder for holding the value.

Definition: var Firstdemo;

Ii. Scope of variables

2.1 Basic Concepts

Use Var to define a variable: A local variable that defines the scope of the variable, and the method that defines the variable is also declared explicitly.

If you don't understand that, you can take a look at this simple, rough example:

Test ();
function Test () {
var firstdemo= "Hello"; // Defining Local Variables
    alert (Firstdemo); // Hello

}

function Test () {    var firstdemo= "Hello";   define local variable Firstdemo              }     alert (Firstdemo); // error, 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: The variable can be accessed globally whenever a function that defines the variable is called once. This method of defining variables is also an implicit declaration

<script type= "Text/javascript" >        Test ();        alert (Firstdemo);   Hello         function  Test () {             Firstdemo= "Hello";                    }             </script>

tips: explicitly declared variables are compiled into the calling object at precompilation, such as Var t=1, var t on precompilation, execution of t=1 on interpretation, or a global variable when an implicit declaration variable is interpreted.

Figuring out the scope of a variable can help us think about how to reasonably declare a variable, which reduces unnecessary memory overhead while greatly avoiding the hassle of overwriting a previously defined variable with a duplicate definition of a variable.

2.2 Scope Analysis

<script type= "Text/javascript" >    function  demofunction (s) {         Document.writeln (s)     }      var //Definition     function Test () {         demofunction (i);         function innerfunction () {             var//                    }         Innerfunction ();             }    Test ();     </script>

Output results: 0 1 0

<script type= "Text/javascript" >

var i=0;
function Test () {
Demofunction (i);
function Innerfunction () {
Demofunction (i);
var I=1;
Demofunction (i);
}
Innerfunction ();
Demofunction (i);
}
Test ();

</script>

Output Result:

A, 0 0 1 0

B, 0 undefined 1 0

C, 0 error I am not defined

You can guess what the result is, the reason will be in the message to explain.

Learn JavaScript from the beginning (ii)--variables and their scopes