Learn JavaScript three (variable) from scratch

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.

To use the var operator when defining variables

Such as:

var message; /* defines a variable named message that can be used to hold any value, and the uninitialized variable will hold a special value-undefined */

second, local variables

You can declare a variable and initialize it simultaneously.

function Test () {    var message= ' Hello ';  // defines a local message variable with an initial value of Hello    alert (message);       // Pop the message content     }    Test (); </script>

The wrong wording

function Test () {    var message= ' Hello ';  // defines a local message variable with an initial value of Hello         }    Test (); alert (message);        // Error

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.

third, define global variables

function Test () {   message= ' Hello ';  // Global message variable with an initial value of Hello         }    Test (); alert (message);        // Hello

Because the Var is omitted here, the message becomes a global variable, so as long as the test () function is called once, the variable is defined and can be accessed anywhere outside the function.

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

iv. scope and scope

<script type= "Text/javascript" >//define an output functionfunctionOutPut (s) {Document.writeln (s);}//Global VariablesvarI=0; //Defining external functionsfunctionouter () {//Accessing global VariablesOutPut (i);//0//define a class-part functionfunctioninner () {//Defining local Variablesvari = 1; //I=1: If implicit declaration is used, then the global variable i is overwrittenOutPut (i);//1} inner (); OutPut (i); //0} outer (); </SCRIPT>

Results: 0,1,0

<script type= "Text/javascript" > function  demofunction (s) {Document.writeln (s)}  Var i=0; 　　　　 function  Test () {demofunction (i); 　　　　　　 function  innerfunction () {demofunction (i); 　　　　 var i=1;　　　　Demofunction (i); 　　　　} innerfunction (); Demofunction (i);} Test (); </script>

Results: 0 undefined 1 0

The local variables declared in the JS function body are valid throughout the function, so var i = 1 in the code above, and in the inner function, in fact the explicitly declared variable i is compiled into the calling object at precompiled time, unlike an implicitly declared variable that is defined as a global variable at the time of interpretation. Just when the output (i) is called, the variable is not initialized, and the local variable i is an unassigned variable, not an undefined variable, so the undefined is output. The above code is equivalent to the following code:

functionvar////I=1//

Learn JavaScript three (variable) from scratch