Javascript [Electronic resources] : The Definitive Guide (4th Edition) نسخه متنی

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.