Which of the following type of variable is visible everywhere in your code?

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A variable is a name associated with a value; we say that the variable stores or contains the value. Variables allow you to store and manipulate data in your programs. For example, the following line of JavaScript assigns the value

var sum = i + 3;

7 to a variable named

var sum = i + 3;

8:

i = 2;

And the following line adds

var sum = i + 3;

9 to

var sum = i + 3;

8 and assigns the result to a new variable,

i = 10;
i = "ten";

1:

var sum = i + 3;

These two lines of code demonstrate just about everything you need to know about variables. However, to fully understand how variables work in JavaScript, you need to master a few more concepts. Unfortunately, these concepts require more than a couple of lines of code to explain! The rest of this chapter explains the typing, declaration, scope, contents, and resolution of variables. It also explores garbage collection and the variable/property duality.[]

An important difference between JavaScript and languages such as Java and C is that JavaScript is untyped. This means, in part, that a JavaScript variable can hold a value of any datatype, unlike a Java or C variable, which can hold only the one particular type of data for which it is declared. For example, it is perfectly legal in JavaScript to assign a number to a variable and then later assign a string to that variable:

i = 10;
i = "ten";

In C, C++, Java, or any other strongly typed language, code like this is illegal.

A feature related to JavaScript’s lack of typing is that the language conveniently and automatically converts values from one type to another, as necessary. If you attempt to append a number to a string, for example, JavaScript automatically converts the number to the corresponding string so that it can be appended. Datatype conversion is covered in detail in Chapter 3.

JavaScript is obviously a simpler language for being untyped. The advantage of strongly typed languages such as C++ and Java is that they enforce rigorous programming practices, which makes it easier to write, maintain, and reuse long, complex programs. Since many JavaScript programs are shorter scripts, this rigor is not necessary, and we benefit from the simpler syntax.

Before you use a variable in a JavaScript program, you must declare it.[] Variables are declared with the

i = 10;
i = "ten";

2 keyword, like this:

var i;
var sum;

You can also declare multiple variables with the same

i = 10;
i = "ten";

2 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

i = 10;
i = "ten";

2 statement, the variable is declared, but its initial value is

i = 10;
i = "ten";

5 until your code stores a value into it.

Note that the

i = 10;
i = "ten";

2 statement can also appear as part of the

i = 10;
i = "ten";

7 and

i = 10;
i = "ten";

8 loops [introduced in Chapter 6], allowing you to succinctly declare the loop variable as part of the loop syntax itself. For example:

for[var i = 0; i < 10; i++] document.write[i, ">brbrbrbrbrbrbrbrbrbrbrbrbrbrbrbrbrbrbrbrbrbrbrbrbrbrbrbrbrbr