foo.js

function addANumberToAthing(a) {

  function doSomethingWithItInScope(b) {
    return a + b;
  }

  return [1, 2, 3].map(doSomethingWithItInScope);
}


// versus


function doSomethingWithItInScope(b) {
  return function(a) {
    return a + b;
  };
}

function addANumberToAthing(a) {
  return [1, 2, 3].map(doSomethingWithItInScope(a));
}