• JavaScript Scope and Context
  • MDN: Hoisting
  • MDN: var
  • MDN: let
  • MDN: function
  • MDN: arrow function
• Functional Array Methods
  • MDN: Array.forEach
  • MDN: Array.map
  • MDN: Array.filter
  • MDN: Array.reduce

• Work in a fork of this repository
• Work in a branch on your fork
• Write all of your code in a directory named lab- + <your name> e.g. lab-susan
• Open a pull request to this repository
• Submit on canvas a question and observation, how long you spent, and a link to your pull request

Given the code blocks below, answer the set of questions below. Please copy the questions to your lab directory in a file called ANSWERS.md.

Explain how hoisting allows the printGreeting function to be called before where it's actually written in the file.

printGreeting();

function printGreeting() {
  console.log("isn't JavaScript wonderful?");
}

Explain why this function called printGoodbye can't be executed until after it's actually written in the file.

printGoodbye(); // this one won't execute!

const printGreeting = () => {
  console.log("That's all, folks!");
}

printGoodbye();

Manually implrement the functional programming functions .forEach, .map, .filter, .reduce. Put them in one module called fp that you can import and use in other modules.

• .forEach - executes a function for each item in a collection. Returns nothing.
• .map - turns one list into a new list by passing each element through the callback function and saving each result in the new list.
• .filter - takes all the elements in a list and returns a new list filtered down to only elements that pass some test defined by the callback function.
• .reduce - takes all the elements in a list and reduces them down to a single value, starting from some initial value.

Create a NodeJS module in the lib/ directory named fp.js that exports an object.

• Create functions called forEach, map, filter, and reduce.
• Each function should accept two parameters:
  • An array of elements
  • A reference to a callback function
  • (reduce actually accepts an additional parameter too.)
• Define each function using ES6 lexical arrow function syntax.
• Do not use any third party librarys in the FP module.

// array: an array of elements
// cb: a callback function
const myFunction = (array, cb) => {

}

• fp.forEach should have the function signature (array, callback) => undefined
• fp.map should have the function signature (array, callback) => Array
• fp.filter should have the function signature (array, callback) => Array
• fp.reduce should have the function signature (array, callback, initialState) => value

Create a NodeJS module in the test directory named fp.test.js that asserts the correctness of the fp module.

• Use Behavior-driven Development describe and test methods to define descriptive tests and increase readablity
• Each test callback should aim to test a small well defined feature of a function
• Write tests to ensure the fp module functions returns the correct results when invoked with valid arguments

const array = [1, 2, 3, 4, 5];
fp.forEach(array, el => console.log(el)); // prints 1 2 3 4 5 across many lines
    fp.map(array, el => el * 2);          // returns [2, 4, 6, 8, 10]
 fp.filter(array, el => el % 2 === 0);    // returns [2, 4]
 fp.reduce(array, (a, b) => a + b, 0);    // returns 15

Reduce a list of numbers to one number representing their sum. Start a running value at 0. Now continually add each single number to the running value. It all reduces down to one number: the total sum of all the numbers together.

let numbers = [1,2,3,4,5];
let sum = fp.reduce(numbers, (runningValue, number) => runningValue + number, 0)

Reduce a budget and a collection of expenses down to how much money is left by starting a running total equal to the budget, then continually subtracting each budget item from the running total. The list of expenses reduces down to just one number: the money left in the budget.

let beginningBudget = 100;
let expenses = [12, 43, 18];
let budgetRemaining = fp.reduce(expenses, (remainingBudget, singleExpense) => {
  return remainingBudget - singleExpense
 }, beginningBudget)