setting a default value to an undefined variable in javascript

Question

Ideally I want to be able to write something like:

function a( b ) {
    b.defaultVal( 1 );
    return b;
}

The intention of this is that if b is any defined value, b will remain as that value; but if b is undefined, then b will be set to the value specified in the parameter of defaultVal(), in this case 1.

Is this even possible?

Ive been toying about with something like this:

String.prototype.defaultVal=function(valOnUndefined){
    if(typeof this==='undefined'){
        return valOnUndefined;
    }else{
        return this;
    }
};

But I'm not having any success applying this kind of logic to any variable, particularly undefined variables.

Does anyone have any thoughts on this? Can it be done? Or am I barking up the wrong tree?

Answer 1

Why not use the default operator:

function someF(b)
{
    b = b || 1;
    return b;
}

Job done! Here's more info about how it works.

Just as a side-note: your prototype won't work, because you're augmenting the String prototype, whereas if your variable is undefined, the String prototype doesn't exactly apply.

To be absolutely, super-duper-sure that you're only switching to the default value, when b really is undefined, you could do this:

var someF = (function(undefined)//get the real undefined value
{
    return function(b)
    {
        b = b === undefined ? 1 : b;
        return b;
    }
})(); // by not passing any arguments