JavaScript Variable Scope
JavaScript Variable Scope
The scope of a variable is the region of your program in which it is defined. JavaScript variables have only two scopes.
- Global Variables− A global variable has global scope which means it can be defined anywhere in your JavaScript code.
- Local Variables− A local variable will be visible only within a function where it is defined. Function parameters are always local to that function.
Within the body of a function, a local variable takes precedence over a global variable with the same name. If you declare a local variable or function parameter with the same name as a global variable, you effectively hide the global variable. Take a look into the following example.
<html>
<body onload = checkscope();>
<script type = “text/javascript”>
<!–
var myVar = “global”; // Declare a global variable
function checkscope( ) {
var myVar = “local”; // Declare a local variable
document.write(myVar);
}
//–>
</script>
</body>
</html>
This produces the following result −
local
JavaScript Variable Names
While naming your variables in JavaScript, keep the following rules in mind.
- You should not use any of the JavaScript reserved keywords as a variable name. These keywords are mentioned in the next section. For example,break or boolean variable names are not valid.
- JavaScript variable names should not start with a numeral (0-9). They must begin with a letter or an underscore character. For example,123testis an invalid variable name but _123test is a valid one.
- JavaScript variable names are case-sensitive. For example,Name andname are two different variables.
JavaScript Reserved Words
A list of all the reserved words in JavaScript are given in the following table. They cannot be used as JavaScript variables, functions, methods, loop labels, or any object names.
| abstract boolean break byte case catch char class const continue debugger default delete do double | else enum export extends false final finally float for function goto if implements import in | instanceof int interface long native new null package private protected public return short static super | switch synchronized this throw throws transient true try typeof var void volatile while with |
