Figuring out how to pass an external context to a JavaScript map function. Why? Say you want a function that operates element-wise on an array, but generates results that it needs to add to some outside object, like an array or a dictionary.

usingMapWithAnExternalContext.js

var myDictionary = {
  rocket: 'Atlas V',
  spaceport: 'Cape Canaveral',
  astronauts: [
    'Brian',
    'Wil'
  ]
};

var getValueFromExternalDictionary = function (currentKey, indexOfCurrentKey) {
  'use strict';
  return (this[currentKey]) // the value of "this" will be supplied as an external context in
                            // the .map() call
};

var keysOfInterest = [
  'rocket',
  'astronauts'
];
keysOfInterest.map(getValueFromExternalDictionary, myDictionary);

// ^ Will return ["Atlas V", Array[2]0: "Brian"1: "Wil"length: 2__proto__: Array[0]]

/* 
Below we change the keys of interest and change their order.
This case is to test whether the function is just chugging along the array,
or whether it's actually performing the lookup in the dictionary we supplied as
an external context
*/
keysOfInterest = [
  'spaceport',
  'rocket'
];

keysOfInterest.map(getValueFromExternalDictionary, myDictionary);

// returns ["Cape Canaveral", "Atlas V"]