Providing a default string parameter in JavaScript

Question

I've always used the following syntax to ensure that the input variable isn't null.

function f(input){
  if(input === null)
    input = "";
  ...
}

Lately, I noticed that it's shorter to express it as follows.

function f(input){
  input = input ? input : "";
  ...
}

But also, I've seen this syntax.

function f(input){
  input = input || "";
  ...
}

1. Are those equivalent (not in what they do but in how they do it)?
2. Which is most recommended (readability etc.)?

Note that I'll be strictly working with inputs of strings such that it's either valid one or null (not provided at all). If I'd like to extend the protection to include other types, what additional issues should I take into consideration?

Answer 1

First, note that they aren't all the same. #2 and #3 have precisely the same result: unless input is truthy, set it to an empty string. #1 means says "unless input is null, set it to an empty string. So if false was provided, input would be false in #1 but '' in #2 and #3.

If nothing was provided to #1, (i.e. f(), input would be undefined, because that is the value that is passed when a parameter is missing, not null. One of these would fix this:

if(input === undefined)
if(input == null)

Otherwise, yes, they are functionally equivalent. For me, the best way to write it would be

input = input || "";

That is my preferred style. It may be yours; it may not. But, above all, be consistent.