Javascript Ternary Operator vs. ||

Question

I was taking a look at some node.js code earlier, and I noticed that the guy who wrote it seemed to favour the following syntax:

var fn = function (param) {
    var paramWithDefault = null == param ? 'Default Value' : param;
}

Over what I consider to be the more concise:

var fn = function (param) {
    var paramWithDefault = param || 'Default Value';
}

I was wondering if the second form is actually more socially acceptable JavaScript syntax, I've seen it out in the wild more times than the ternary operator for this purpose.

I note that in the first example he's using the double equals (not the triple equals) which means it will count "undefined" as null, which would reduce one impact that I could think of. However, I've read in numerous places that == is a rather evil operator in JavaScript (JSLint is very much against it, IIRC).

Answer 1

Because this code would evaluate to 'Default Value' everytime you passed in 0, "", false, or some other falsy value.

function fn(param) {
  var paramWithDefault = param || 'Default Value';
  return paramWithDefault;
}

It might not bite you on how you use this particular function, but it is a bad pattern to avoid when you do care about passing in things like empty strings or 0 or a boolean.

Answer 2

What you really need is a null coalesce operator. But seeing that javascript doesn't really have one, programmers typically use '||' to stand in for it.

However, both are perfectly reasonable. To those that don't understand what a null coalesce operator is, the ternary operator is probably more likely to be understood.