I try not to be too pedantic about grammar, but I have a few pet peeves. It bothers me when people use less instead of fewer, and don't get me started on the overuse of literally.

In JavaScript, it feels like nails on a chalkboard whenever I see a conditional which redundantly evaluates to true or false. Take this example:

var isEmpty = function (array) {
  if (array.length === 0) {
    return true;
  } else {
    return false;
  }
};

Because isEmpty should return true or false, the developer did exactly that. However, he failed to realize that the condition itself is already a boolean. The same function could be rewritten as:

var isEmpty = function (array) {
  return array.length === 0;
};

Here's an example where the condition is not already a boolean:

var docExists = function (id) {
  if (Collection.findOne(id)) {
    return true;
  } else {
    return false;
  }
};

In this situation we should use a !! to coerce the result of findOne:

var docExists = function (id) {
  return !!Collection.findOne(id);
};

And that's it! Now go write fewer lines of code.