4.1 Variable Typing

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 data type, 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. We'll see more about data type conversion in Chapter 11.

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.