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method overloading

P: n/a
can we overload a javascript function with different argument?

example:

function a(a){}
function a(a,b){}
Jul 23 '05 #1
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4 Replies


P: n/a
John Smith wrote:
can we overload a javascript function with different argument?

example:

function a(a){}
function a(a,b){}


See for yourself:

<script type="text/javascript">
function doalert(a, b) {
alert('doalert#1 called');
}

function doalert(a) {
alert('doalert#2 called');
}

doalert(1,2);
</script>

You will see that the second definition of the doalert() function overrides
the first one, so the answer is no.

An alternative would be to use a variable number of arguments when calling a
function and decide what to do based upon the defined arguments.
JW

Jul 23 '05 #2

P: n/a
On 28/03/2005 09:31, John Smith wrote:
can we overload a javascript function with different argument?

example:

function a(a){}
function a(a,b){}


Not in the way other languages define method overloading, no. In the
case above, the function object created by the first function
declaration would immediately be replaced by the second.

There are a few approaches that can be useful, depending on what
you're trying to achieve. You give no details so I'll provide a quick
run through them and you can determine which is best. I'll apologise
now for the exceedingly artifical examples. :/

1) Optional arguments

If a method is defined as taking optional arguments, you can add
the defaults by taking advantage of ECMAScript's rather different
logical OR (||) operator:

function myFunction(a, b) {
b = b || 'default';
}

If the argument, b, evaluates to false (that is, zero (0), empty
string (''), null, undefined, or false), the second operand will
be assigned in its place.

If one of these "false" values is legal, but not the default, see
(3) for an alternative.

2) Overloading by number of arguments

The arguments object available within all functions contains all
arguments passed to the function as properties, and the number of
arguments in its length property. You can use the value of the
latter to change the behaviour of the function, and the former to
access any arguments that do not have a corresponding formal
identifier.

function myFunction() {var msg;
if(arguments.length) {
msg = arguments.length + ' arguments passed:\n';

for(var i = 0, n = arguments.length; i < n; ++i) {
msg += '\n' + arguments[i];
}
} else {
msg = 'No arguments passed!';
}
alert(msg);
}

3) Overloading by type of arguments

I sometimes use this approach when I want to allow a function to
take either a reference to an element, or its id attribute value.
You can examine the type of the argument using the typeof
operator:

function myFunction(element) {
if('string' == typeof element) {
element = document.getElementById(element);
}
if(element) {
/* ... */
}
}

If a function is called and an argument doesn't have a corresponding
value, it will be undefined with typeof evaluating to the string,
'undefined'. Returning to the example in (1), you might have an
argument which expects an integer. If the argument was unspecified,
you might want it to assume the value -1, but zero is legal. In that
case,

function myFunction(a, b) {
if('undefined' == typeof b) {b = -1;}
}
Hope that helps,
Mike

--
Michael Winter
Replace ".invalid" with ".uk" to reply by e-mail.
Jul 23 '05 #3

P: n/a
JRS: In article <Jw****************@text.news.blueyonder.co.uk>, dated
Mon, 28 Mar 2005 15:37:13, seen in news:comp.lang.javascript, Michael
Winter <m.******@blueyonder.co.invalid> posted :
1) Optional arguments

If a method is defined as taking optional arguments, you can add
the defaults by taking advantage of ECMAScript's rather different
logical OR (||) operator:

function myFunction(a, b) {
b = b || 'default';
}

If the argument, b, evaluates to false (that is, zero (0), empty
string (''), null, undefined, or false), the second operand will
be assigned in its place.


That means that if the default is not zero, etc., it is impossible to
give and get used a parameter which is zero, etc.

How about if (b==null) b = 'default' ? It should enable having a
default which is not zero, etc., and giving p parameter which is zero,
an empty string, undefined, or false - although not for a value which,
like that of var U , is undefined, which needs b===null .

--
John Stockton, Surrey, UK. ?@merlyn.demon.co.uk Turnpike v4.00 IE 4
<URL:http://www.jibbering.com/faq/> JL/RC: FAQ of news:comp.lang.javascript
<URL:http://www.merlyn.demon.co.uk/js-index.htm> jscr maths, dates, sources.
<URL:http://www.merlyn.demon.co.uk/> TP/BP/Delphi/jscr/&c, FAQ items, links.
Jul 23 '05 #4

P: n/a
On 28/03/2005 21:20, Dr John Stockton wrote:

[MLW:]
function myFunction(a, b) {
b = b || 'default';
}

[snip]
That means that if the default is not zero, etc., it is impossible to
give and get used a parameter which is zero, etc.
I thought I made that point (or at least a similar one) at the end of
that section?
How about if (b==null) b = 'default' ?
My later suggestion was the use of the typeof operator that, whilst
slower than a equality test with null, should be foolproof for all
situations.
not for a value which, like that of var U , is undefined, which needs b===null .


As null and undefined are of different types, undefined !== null.
You'd either want to use typeof or a strict comparison with an
uninitialised variable. Of course, if you don't care about early
browsers and pre-IE 5.5 (not likely), you could also use the undefined
keyword.

Mike

--
Michael Winter
Replace ".invalid" with ".uk" to reply by e-mail.
Jul 23 '05 #5

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