We say a language has "first-class functions" if it supports functions being passed as input or output values of other functions. JS has this feature, and JavaScripters take advantage of it all the time - e.g. it's what allows us to pass a callback function as an input parameter for another function. It's also possible to have a function as a return value. A function which either takes or returns another function is called a "higher-order function".

• Run the tests in filterMapReduce.test.js with the command node filterMapReduce.test.js - tests should be failing
• Work through the TODOs in filterMapReduce.js to write recursive implementations of all three functions
  • To move away from the object-oriented method-calling mindset to a functional mindset, use the helper functions head, tail, and count defined in arrayHelpers.js
  • Run the tests again - most should now be passing
• Work through the TODOs in filterMapReduce.test.js to write applications of these functions that get the remaining tests to pass
• Compare your implementations with those of your partner/group and discuss