JavaScript
Function Parameters
A JavaScript
function does not perform any checking on parameter values (arguments).
Function
Parameters and Arguments
Earlier in
this tutorial, you learned that functions can have parameters:
functionName(parameter1,
parameter2, parameter3) {
code to be executed
}
Function
parameters are the names listed in the function definition.
Function
arguments are the real values passed to (and received by) the function.
Parameter
Rules
JavaScript
function definitions do not specify data types for parameters.
JavaScript
functions do not perform type checking on the passed arguments.
JavaScript
functions do not check the number of arguments received.
Parameter
Defaults
If a
function is called with missing arguments (less than declared), the missing
values are set to: undefined
Sometimes
this is acceptable, but sometimes it is better to assign a default value to the
parameter:
Example
function
myFunction(x, y) {
if (y === undefined) {
y = 0;
}
}
»
If a function
is called with too many arguments (more than declared), these arguments can be
reached using the arguments object.
The
Arguments Object
JavaScript
functions have a built-in object called the arguments object.
The argument
object contains an array of the arguments used when the function was called
(invoked).
This way you
can simply use a function to find (for instance) the highest value in a list of
numbers:
Example
x
= findMax(1, 123, 500, 115, 44, 88);
function
findMax() {
var i;
var max = -Infinity;
for (i = 0; i < arguments.length; i++) {
if (arguments[i] > max) {
max = arguments[i];
}
}
return max;
}
»
Or create a
function to sum all input values:
Example
x
= sumAll(1, 123, 500, 115, 44, 88);
function
sumAll() {
var i, sum = 0;
for (i = 0; i < arguments.length; i++) {
sum += arguments[i];
}
return sum;
}
»
Arguments
are Passed by Value
The
parameters, in a function call, are the function's arguments.
JavaScript
arguments are passed by value: The function only gets to know the values, not
the argument's locations.
If a
function changes an argument's value, it does not change the parameter's
original value.
Changes to
arguments are not visible (reflected) outside the function.
Objects are
Passed by Reference
In
JavaScript, object references are values.
Because of
this, objects will behave like they are passed by reference:
If a
function changes an object property, it changes the original value.
Changes to
object properties are visible (reflected) outside the function
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