Idiomatic way to set default value in JavaScript

Question

What would be the most idiomatic way to do the following in JavaScript:

If myParam is not passed into MyFunc by the caller, then I want to set it to a default value. But first I want to try and get it from another object, which may not yet exist:

function MyFunc(myParam) {

    if (!myParam) {
        if (!myObj) {
            myParam = 10;
        }
        else {
            myParam = myObj.myParam;
        }
    }
 
    alert(myParam);
}

I started to write:

myParam = myParam || myObj.mParam || 10

but realized that if myObj does not exist then this would fail. I might guess the following:

myParam = myParam || (myObj && myObj.mParam) || 10

It might even work. But is it the best way?

How would, for example, John Resig do it?

Answer 1

If myObj is a global it needs to reference the window object, otherwise it will throw an error if myObj is undefined.

myParam = myParam || (window.myObj ? window.myObj.mParam : 10);

or

myParam = myParam || (window.myObj && window.myObj.mParam) || 10;

This works as well:

myParam = myParam || ((typeof myObj !== "undefined") ? myObj.mParam : 10);

Answer 2

I think the other answers have proven that there are a whole bunch of ways a one-liner can fail here. My version below is perhaps more readable, and doesn't explicitly test for object/property existence, but isn't much shorter:

function MyFunc(myParam){
    if (!myParam){
        try{
            myParam = myObj.myParam;
        }
        catch(e){
            myParam = 10;
        }
    }
}