Question

For architecture and namespacing purposes, I want to do this:

function outer (arr) {
  function inner(arrElement) {
    return doStuffTo(arrElement); 
  }
  var results = [];

  arr.forEach(element, index, array) {
    results.push(inner(element));
  }
  return results;
}

So basically, a function within a function. Simple stuff. But outer() is something that will be executed a lot. Does this mean the overhead of defining a function (on top of evaluating it) will apply every time outer() is called? For this to be efficient, must I define inner() outside?

Answer 1

You could use a closure:

var outer = (function() {

  function inner(arrElement) {
    return doStuffTo(arrElement);
  }

  return function (arr) {
    var results = [];
    arr.forEach(element, index, array) {
      results.push(inner(element));
    }
    return results;
  }
}());

inner is held in a closure and remains “private” to outer, and is only created once when outer is initialised.