What is the most efficient method of setting default parameter values in javascript?

Question

I know of two methods of setting a default parameter, but I would like to know what the preferred method is.

function Foo(par1, par2)
{
    if(par2 == null)
        par2 = "my default"
}

or

function Foo(par1, par2)
{
    par2 = par2 || "my default"
}

or is there a better way than either of these?

EDIT:

I would also like to know how others handle multiple optional parameters such as this: We have several functions like this in internal libraries (I think they're pretty ugly).

function Foo(par1, par2, par3)
{
    if(par2 == null)
        par2 = "my default"
    if(par3 == null)
        par3 = "my default"
    // Do something
}

And to call it:

Foo("Parameter one",null,true)

Answer 1

I usually use the second one because it adds less signal.

Answer 2

Never heard of the second way you mentioned, that is interesting. I would do:

function Foo(par1, par2)
{
    par2 = par2 ? par2 : 'default value';
}

But now that you bring to my attention your second method, I think I would use that just because it is less typing.

Answer 3

If you use jquery, you can do:

function foo(params){
 var params = $.extend({}, {bar: '2'}, params);
 alert(params.bar);
}
foo(); //Alerts 2

Answer 4

first one is actually wrong, since they would be undefined, not null.

par2 !== null would return true for this case.

since javascript, if loosely compared, compare null, undefined, false to the same value, I would suggest checking explicitly for an undefined value.

if (par2 !== undefined)
        par2 = "my default";

or

    par2 = par2 !== undefined ? par2 : "my default";

This will allow you to pass on values like false or null.

However, your second approach is convenient, but only in the case you know that you would never pass false or null.

Answer 5

Similar and related questions have been asked before.

Answer 6

The choice I make depends on the parameter type and the default value required.

For example, this will assign the 'default value' if par2 is false, 0, an empty string, null or undefined:

par2 = par2 || 'default value';

That behavior may not be what is expected or required.

Answer 7

Personally I think the second form is just outright cleaner and more readable, certainly moreso than sergey's arguments inspection - though that can have it's place - but I prefer to pass extensible objects for args than have to maintain arg signatures.

e.g.

function Foo(myArgs)
{
    myArgs.par1 = myArgs.par1 || "my default"
    myArgs.par2 = myArgs.par2 || "my default"
}

Answer 8

The one thing you have to think about when using the solution of

var foo = bar || 123;

is what values of bar that evaluate to false. It could cause you problems down the road.

Eric

Answer 9

If you're passing many arguments to a function you could do something like:

function myFunc() {
    arguments[0] = arguments[0] || "default value for first argument";
    arguments[3] = arguments[3] || "default value for fourth argument";
    alert(arguments[3]);
}

Answer 10

What about this one:

function myFunc(arg1, arg2, arg3) {
    switch (arguments.length) {
        case 0 : arg1 = "default1";
        case 1 : arg2 = "default2";
        case 2 : arg3 = "default3";
    }
}