[Chapter 5] 5.10 var
Author: David Flanagan
Date: Fri Aug 29 10:52:39 1997

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| Chapter 5 Statements |

## 5.10 var

We saw the `var` statement in Chapter 3, Variables and Data Types; it provides a way to explicitly declare a variable or variables. The syntax of this statement is:

    var name_1 [ = value_1] [ ..., name_n [= value_n]]

That is: the `var` keyword is followed by a variable name and an optional initial value, or it is followed by a comma-separated list of variable names, each of which can have an initial value specified. The initial values are specified with the `=` operator and an arbitrary expression. For example:

    var i;
    var j = 0;
    var x = 2.34, y = 4.12, r, theta;

If no initial value is specified for a variable with the `var` statement, then the variable will be defined, but its initial value will be the special JavaScript undefined value.

The `var` statement should always be used when declaring local variables within functions. Otherwise, you run the risk of overwriting a top-level variable of the same name. For top-level variables, the `var` statement is not required. Nevertheless, it is a good programming practice to use the `var` statement whenever you create a new variable. It is also a good practice to group your variable declarations together at the top of the program or at the top of a function.

Note that the `var` statement can also legally appear as part of the `for` and `for/in` loops, in order to declare the loop variable as part of the loop itself. For example:

    for(var i = 0; i < 10; i++) document.write(i, "<BR>");
    for(var i = 0, j=10; i < 10; i++,j--) document.write(i*j, "<BR>");
    for(var i in o) document.write(i, "<BR>");

A variable declared in a loop in this way is not local to the loop as it would be in C++ or Java; its scope is the same as it would be if it had been declared outside of the loop.

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