JavaScript
Function Definitions
JavaScript
functions are defined with the function keyword
You can use
a function declaration or a function expression.
Function
Declarations
Earlier in
this tutorial, you learned that functions are declared with the following
syntax:
function
functionName(parameters) {
code to be executed
}
Declared
functions are not executed immediately. They are "saved for later
use", and will be executed later, when they are invoked (called upon).
Example
function
myFunction(a, b) {
return a * b;
}
»
Semicolons
are used to separate executable JavaScript statements.
Since a
function declaration is not an executable statement, it is not common to end it
with a semicolon.
Function
Expressions
A JavaScript
function can also be defined using an expression.
A function
expression can be stored in a variable:
Example
var
x = function (a, b) {return a * b};
»
After a
function expression has been stored in a variable, the variable can be used as
a function:
Example
var
x = function (a, b) {return a * b};
var
z = x(4, 3);
»
The function
above is actually an anonymous function (a function without a name).
Functions
stored in variables do not need function names. They are always invoked
(called) using the variable name.
The function
above ends with a semicolon because it is a part of an executable statement.
The
Function() Constructor
As you have
seen in the previous examples, JavaScript functions are defined with the
function keyword.
Functions
can also be defined with a built-in JavaScript function constructor called
Function().
Example
var
myFunction = new Function("a", "b", "return a *
b");
var
x = myFunction(4, 3);
»
You actually
don't have to use the function constructor. The example above is the same as
writing:
Example
var
myFunction = function (a, b) {return a * b};
var x = myFunction(4, 3);
»
Most of the
time, you can avoid using the new keyword in JavaScript.
Function
Hoisting
Earlier in
this tutorial, you learned about "hoisting".
Hoisting is
JavaScript's default behavior of moving declarations to the top of the current
scope.
Hoisting
applies to variable declarations and to function declarations.
Because of
this, JavaScript functions can be called before they are declared:
myFunction(5);
function
myFunction(y) {
return y * y;
}
Functions
defined using an expression are not hoisted.
Self-Invoking
Functions
Function
expressions can be made "self-invoking".
A
self-invoking expression is invoked (started) automatically, without being
called.
Function
expressions will execute automatically if the expression is followed by ().
You cannot
self-invoke a function declaration.
You have to
add parentheses around the function to indicate that it is a function
expression:
Example
(function
() {
var x = "Hello!!"; // I will invoke myself
})();
»
The function
above is actually an anonymous self-invoking function (function without name).
Functions
Can Be Used as Values
JavaScript
functions can be used as values:
Example
function
myFunction(a, b) {
return a * b;
}
var
x = myFunction(4, 3);
»
JavaScript
functions can be used in expressions:
Example
function
myFunction(a, b) {
return a * b;
}
var
x = myFunction(4, 3) * 2;
»
Functions
are Objects
The typeof
operator in JavaScript returns "function" for functions.
But,
JavaScript functions can best be described as objects.
JavaScript
functions have both properties and methods.
The
arguments.length property returns the number of arguments received when the
function was invoked:
Example
function
myFunction(a, b) {
return arguments.length;
}
»
The
toString() method returns the function as a string:
Example
function
myFunction(a, b) {
return a * b;
}
var
txt = myFunction.toString();
»
A function
defined as the property of an object, is called a method to the object.
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