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

6.14 function

The

function
statement defines a JavaScript function. It has the following syntax:

function funcname([arg1 [,arg2 [..., argn]]]) {
statements
}

funcname is the name of the function being
defined. This must be an identifier, not a string or an expression.
The function name is followed by a comma-separated list of argument
names in parentheses. These identifiers can be used within the body
of the function to refer to the argument values passed when the
function is invoked.

The body of the function is composed of
any number of JavaScript statements, contained within curly braces.
These statements are not executed when the function is defined.
Instead, they are compiled and associated with the new function
object for execution when the function is invoked with the (
) function call operator. Note that
the curly braces are a required part of the
function statement. Unlike statement blocks used
with while loops and other statements, a function
body requires curly braces, even if the body consists of only a
single statement.

A function definition creates a new function object and stores that
object in a newly created property named
funcname. Here are some example function
definitions:

function welcome(  ) { alert("Welcome to my home page!"); }
function print(msg) {
document.write(msg, "<br>");
}
function hypotenuse(x, y) {
return Math.sqrt(x*x + y*y);  // return is documented below
}
function factorial(n) {           // A recursive function
if (n <= 1) return 1;
return n * factorial(n - 1);
}

Function definitions usually appear in top-level JavaScript code.
They may also be nested within other function definitions, but only
at the "top level" of those functions; that is, function
definitions may not appear within if statements,
while loops, or any other statements.

Technically speaking, the function statement is
not a statement. Statements cause dynamic behavior in a JavaScript
program, while function definitions describe the static structure of
a program. Statements are executed at runtime, but functions are
defined when JavaScript code is parsed, or compiled, before it is
actually run. When the JavaScript parser encounters a function
definition, it parses and stores (without executing) the statements
that comprise the body of the function. Then it defines a property
(in the call object if the function definition is nested in another
function; otherwise, in the global object) with the same name as the
function to hold the function.

The fact that function definitions occur at parse time rather than at
runtime causes some surprising effects. Consider the following code:

alert(f(4));     // Displays 16. f(  ) can be called before it is defined.
var f = 0;       // This statement overwrites the property f.
function f(x) {  // This "statement" defines the function f before either
return x*x;  // of the lines above are executed.
}
alert(f);        // Displays 0. f(  ) has been overwritten by the variable f.

These unusual results occur because function definition occurs at a
different time than variable definition. Fortunately, these
situations do not arise very often.

We'll learn more about functions in Chapter 7.