How to set default value for function parameter?

[kefeso]

Hi everyone,

I am rather new to javascript. I've been searching now for quite a while trying to look for a way to give the default value to a function argument if it's not given. For example, in php one would do the follow:

function func_name(foo = 'default') {
...
} 

In the above case, if foo isn't supplied while calling for the function, then foo would default to 'default'. I have tried that in javascript, but it doesn't appear to be working, so I am wondering how should I do this in javascript? Thanks

[Kravvitz]

No, JavaScript doesn't support that PHP way to do it.

How to give default values for function arguments in JavaScript.

[MikeFoster]

Another "default value" question? Something smells fishy

[Pepejeria]

This must in the top 3 asked questions this month

[mrhoo]

function func_name(foo) {
foo= foo||default;

}

[Kravvitz]

Yes, that's one way to do it, mrhoo. However, that doesn't work if you want foo to be able to have a value that evaluates to false.

Yes, this question has been asked a lot in the last few weeks.

[mrhoo]

Oops.

function func_name(foo) {
foo= (foo != undefined)? foo : default;

}

0, false, NaN and the empty string ('') are each not equal to (!=) undefined .
The same values, plus null are not equivilent to (!==) undefined.
To coerce a default value only for func() and func(undefined):

function func(foo){
foo= (foo!== undefined)? foo: 'default';
}

[Kravvitz]

mrhoo, don't you mean the following?

function func_name(foo) {
  foo= (tyepof(foo)!='undefined')? foo : 'default value';

}

[kefeso]

Wow I didn't realize that this question has been asked so much in the past while. MikeFoster, I can however guarantee you that there isn't a conspiracy on my side, don't know about others though

Thanks for everyone's reply. The method actually worked. Oddly though, it was what I used, the only reason why is didn't work before was because I used the function in 'window.onload'. Could anyone explain why I can't give arguments to a function for window.onload please? Thanks a bunch.

[Kravvitz]

Why do you want to give arguments to a window.onload event handler?

Can you show us your code?

[MikeFoster]

Wow I didn't realize that this question has been asked so much in the past while. MikeFoster, I can however guarantee you that there isn't a conspiracy on my side, don't know about others though

[kefeso]

Can you show us your code?

Sure. As a beginner, this is my attempt at creating an active clock.

function proc_time(id) {
	var e = document.getElementById(id); 
	var d = new Date(); 
	var hour = fix_time(d.getHours()); 
	var min = fix_time(d.getMinutes());
	var sec = fix_time(d.getSeconds()); 
	e.innerHTML = '<span id="hour">' + hour + '</span>:<span id="min">' + min + '</span>:<span id="sec">' + sec + '</span>';
}
		
function display_time(check) { 
	check = (check != undefined) ? check : 0; 
	a.innerHTML = check; 
			
	if (check == 0) { 
		proc_time('clock'); 
		a.timeInterval = setInterval(function() { proc_time('clock'); } , 500); 
	} else if (check == 1) { 
		clearInterval(a.timeInterval); 
	} 
} 
		
window.onload = display_time; 

Feel free to point out any beginner's bad coding practices. Help appreciated

[Pepejeria]

mrhoo, don't you mean the following?

function func_name(foo) {
  foo = (tyepof(foo)!='undefined')? foo : 'default value';

}

No need to use the typeof, undefined is a primitive value. Better would have been to check for strict equality though

function func_name(foo) { 
    foo= (foo !== undefined)? foo : "default";
}

[mrhoo]

I wasn't sure if this was implementation dependent, so I checked in
the browsers I have installed.

undefined returns equal to ('==') the undefined keyword,
values that have not been defined, and null.
All other values are not == undefined.

The only change the equivilence ('===') operator enforces is
that it distinguishes null values from undefined.

If you do NOT want to treat null as if it were undefined, use '!==',
otherwise use '!='.

var non=[0, false, NaN ,'',null, undefined];
var tx= '0,false,NaN,"",null,undefined (keyword),'.split(/,/);
var A=['Expressions with undefined',''], i=0, str='';

while(i< 7){
	if(tx[i]===undefined) tx[i]='~Really undefined~';
	A.push('('+tx[i]+ ' != undefined) returns '+ (non[i] != undefined));
	if(non[i] == undefined){
		A.push('and ('+tx[i]+ ' !== undefined) returns '+ (non[i] !== undefined));
	}
	A.push('');
	i++;
}

alert(A.join('\n'))

[Kravvitz]

kefeso, you could use an anonymous function:

window.onload = function(){
  display_time(0);
}

Pepejeria, versions of IE before 5.5 don't seem to support the undefined keyword.

[Pepejeria]

@ Kravvitz, yeah, this was added to IE in version 5.5

[kefeso]

kefeso, you could use an anonymous function

Hi Kravvitz,

Thanks for the advice. However, I am just wondering why couldn't give parameters directly there? I just want to figure out the mechanism behind it. Thanks.

[Kravvitz]

You're welcome

I'll try to explain how it works...

Let's say we have a function that we want to assign as an event handler.

function loadHandler(){
  alert('the page has finished loading');
}

If we use what is shown in the following code block, then the function is executed when this statement is evaluated and whatever value that the function returns will be set as the event handler. (If we had a function that returned a function it would be a different matter.) Whether we pass any arguments in this function call or not doesn't matter.

window.onload = loadHandler();

Now if we use this then we are passing a reference to the function which will be used to call it when the event is triggered.

window.onload = loadHandler;

We could also do the following, which is to use an anonymous function to contain calls to other functions and/or other statements. We set the anonymous function as the event handler so that when the event is triggered the contents of the function will be executed. This also allows us to pass arguments if we want to do so.

window.onload = function(){
  loadHandler();
  alert('hello world');
}

@ Pepejeria, that's why I use typeof instead. It works fine and allows me to support a few more, although very old, browsers.