How best to implement out params in JavaScript?

The callback approach mentioned by @Felix Kling is probably the best idea, but I’ve also found that sometimes it’s easy to leverage Javascript object literal syntax and just have your function return an object on error:

function mightFail(param) {
  // ...
  return didThisFail ? { error: true, msg: "Did not work" } : realResult;
}

then when you call the function:

var result = mightFail("something");
if (result.error) alert("It failed: " + result.msg);

Not fancy and hardly bulletproof, but certainly it’s OK for some simple situations.