handling on no argument in javascript functions

Question

I have little experience in javascript but in other programming languages the following is possible, to set a default value for a missing parameter in a function.

For instance lets say i have a function

function foo(a , b)
  return a + b 
{

I would like to be able to call the above function with 1 parameter or with 2 parameters

one parameter case:

foo(2)   //a =2 and b would be automatically set to a default no argument value,

two parameter case:

foo(1,2)  // a = 1 , b = 2 => 3

Is this possible in javascript that is IE 8 compatible or using Jquery 1.7.2?

score 6 · Accepted Answer · answered Mar 08 '13 at 00:24

6

JavaScript functions allow you to call them with less parameters than specified.

What you're trying to accomplish is possible doing something like the following:

function foo(a , b){
  if(typeof b === "undefined"){
      b=3;
  }
  return a + b; 
}
foo(5);//return 3;

I strongly suggest that you check for undefined and not rely on the || operator. I've spent hours debugging code that made that assumption and failed when passing 0 or another falsy value. The JavaScript type system can get really tricky, for example "0" is falsy.

ECMAScript Harmony (the next version) allows default parameters like in those other languages using the following syntax

function foo(a , b=3){
  return a + b;
}

This does not work in IE8 like the first version though.

answered Mar 08 '13 at 00:24

Benjamin Gruenbaum

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1

This is the best answer, you're the only one who's actually checking for `undefined`... – Elliot Bonneville Mar 08 '13 at 00:26
yeah the undefined check is really needed – pyCthon Mar 08 '13 at 00:33
@pyCthon If you like my answer please consider accepting it, or telling me what parts you did not think are clear enough so I can improve it. – Benjamin Gruenbaum Mar 08 '13 at 01:11
@BenjaminGruenbaum its perfectly clear I just couldn't accept this as an answer with in certain time limit in which i went afk until now – pyCthon Mar 08 '13 at 02:00

Chris Tavares · Answer 2 · 2013-03-08T01:05:32.583

Yes, it's possible. If a parameter isn't passed, you get a special value "undefined". The following works for me:

function foo(a, b) {
  a = a || defaultForA;
  b = b || defaultForB;

}

Due to the way Javascript treats conversion to boolean, the following will all be considered undefined in the above code snippet:

undefined
null
0
"" (empty string)
false

So if those values are important to you, you'll need to explicitly test for undefined rather than the blanket "if not false" test being used above.

Elliot Bonneville · Answer 3 · 2013-03-08T00:40:53.883

3

No, but you can simulate this effect manually with a tertiary operator or an if statement (just make sure you check for undefined!):

function foo(a, b) {
    b = (typeof b === "undefined") ? 2 : b;

    return a + b;
}

edited Mar 08 '13 at 00:40

answered Mar 08 '13 at 00:24

Elliot Bonneville

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score 2 · Answer 4 · answered Mar 08 '13 at 00:24

2

An undefined parameter evaluates as false, so you can do something like this in the beggining of your function foo:

a = a || 0;
b = b || 0;

That way if a or b exist, they will be evaluated with their own values and with 0 otherwise.

answered Mar 08 '13 at 00:24

Héctor Luaces Novo

429
3
11

score 2 · Answer 5 · answered Mar 08 '13 at 00:27

2

You can just test if you received a parameter:

function foo(a, b) {
  if(typeof b === "undefined") {b = 2;}
}

By testing for "undefined" you can also accept parameters that evaluate to false (like null or "" or 0 or false)

answered Mar 08 '13 at 00:27

Steve

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handling on no argument in javascript functions

5 Answers5