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

Well the most important thing to learn here is if a variable is undefined you, by default, do not have any methods available on it. So you need to use a method that can operate on the variable, but is not a member/method of that variable.

Try something like:

function defaultValue(myVar, defaultVal){
    if(typeof myVar === "undefined") myVar = defaultVal;
    return myVar;
}

What you’re trying to do is the equivalent of this:

undefined.prototype.defaultValue = function(val) { }

…Which for obvious reasons, doesn’t work.