Variable Declaration

Before you use a variable in a JavaScript program, you should declare it. Variables are declared with the var keyword, like this:

var i;
var sum;

You can also declare multiple variables with the same var keyword:

var i, sum;

And you can combine variable declaration with variable initialization:

var message = "hello";
var i = 0, j = 0, k = 0;

If you don’t specify an initial value for a variable with the var statement, the variable is declared, but its value is undefined until your code stores a value into it.

Note that the var statement can also appear as part of the for and for/in loops (introduced in Chapter 4), allowing you to succinctly declare the loop variable as part of the loop syntax itself. For example: