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3.9 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 (intro-
duced in Chapter 5), allowing you to succinctly declare the loop variable as part of the
loop syntax itself. For example:
for(var i = 0; i < 10; i++) console.log(i);
for(var i = 0, j=10; i < 10; i++,j--) console.log(i*j);
for(var p in o) console.log(p);
If you’re used to statically typed languages such as C or Java, you will have noticed that
there is no type associated with JavaScript’s variable declarations. A JavaScript variable
can hold a value of any type. For example, it is perfectly legal in JavaScript to assign a
number to a variable and then later assign a string to that variable:
var i = 10;
i = "ten";
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