Faith's FP lesson examples

calculator.js

var lhs = 5;
var rhs = 8;
var operator = "+";
var result;

if (operator === "+") {
  result = lhs + rhs;
} else if (operator === "-") {
  result = lhs - rhs;
} else if (operator === "*") {
  result = lhs * rhs;
} else if (operator === "/") {
  result = lhs / rhs;
}

calculator2.js

var fnDict = {
  "+": function(lhs, rhs) { // normal function
    return lhs + rhs;
  },
  "-": (lhs, rhs) => {      // arrow function
    return lhs - rhs;
  },
  "*": (lhs, rhs) => lhs * rhs, // implied return if no { } s
  "/": (lhs, rhs) => lhs / rhs
};

var lhs = 5;
var rhs = 8;
var operator = "+";
var result;

var fn = fnDict[operator];
result = fn(lhs, rhs);

// or in one line:
// result = fnDict[operator](lhs, rhs);

call-times-2.js

// variable declaration and assignment, where value is a function expression.
let hello = function (subject) {
  console.log("Hello, " + subject + "!");
};

var goodBye = function(subject) {
  console.log("Goodbye, " + subject + "!");
};

// or as an arrow function
// let hello = () => {
//   console.log("Hello");
// };

// vs a function statement
// function hello() {       // a top level named function definition
//   console.log("Hello");  // aka a function statement in JS
// }

function call3TimesWith(fn, subject) {
  fn(subject);
  fn(subject);
  fn(subject);
}

call3TimesWith(hello, "Sarah"); // should call hello("Sarah") 3 times
call3TimesWith(goodBye, "Sarah");

call-times.js

// variable declaration and assignment, where value is a function expression.
let hello = function () {
  console.log("Hello");
};

// or as an arrow function
// let hello = () => {
//   console.log("Hello");
// };

// vs a function statement
// function hello() {       // a top level named function definition
//   console.log("Hello");  // aka a function statement in JS
// }

function call3Times(fn) {
  fn();
  fn();
  fn();
}

call3Times(hello); // should call hello 3 times

example.js

var hello = function() { // rhs is called a function expression
  console.log("Hello");
};

var goodBye = function() {
  console.log("Goodbye");
};

var someFn = hello; // notice we didn't call hello, i.e. hello()

hello();  // calls hello

someFn(); // calls hello

someFn = goodBye;

someFn(); // call goodbye

FP.md

Higher Order Functions

A function is a value, and can be stored in a variable, passed to another function as an argument, or returned as a return value by another function.

map.js

const numbers = [1, 2, 3];

const numbersDoubled = numbers.map(n => n * 2); // [2, 4, 6]

// Break it down:
// const double = n => n * 2;
// const numbersDoubled = numbers.map(double);

// what if we had to do this ourselves?
let numbersDoubled = [];
for (let n of numbers) {
  numbersDoubled.push(n * 2);
}

// what if we had to get the square of each number?
let numbersSquared = [];
for (let n of numbers) {
  numbersSquared.push(n * n);
}

const students = [
  { name: "Faith", favoriteShow: "Doctor Who" },
  { name: "Rachel", favoriteShow: "Star Trek" },
  { name: "Nerando", favoriteShow: "Top Gear" },
  { name: "Kris",  favoriteShow: "Doctor Who" }
];

// Want to get just the names, and then join them together
let names = [];
for (let student of students) {
  names.push(student.name);
}

console.log(names.join(", "));

// Now using array's map (you didn't have to write this template yourself)
let names = students.map((student) => student.name);
console.log(names.join(", "));

// My map works just like array's .map, but instead of
// array.map(fn), you write myMap(array, fn)
// myMap or array.map is like a template, where you only need
// to substitute in one operation within a larger template
function myMap(array, fn) {
  let results = [];
  for (let item of array) {
    results.push(fn(item));
  }
  return results;
}

// This line uses the template:
let names = myMap(students, (student) => student.name);
console.log(names.join(", "));

templates.js

let result = [];
for (let item of array) {
  result.push(n * n);
}

let result = [];
for (let item of array) {
  result.push(student.name);
}

let result = [];
for (let item of array) {
  result.push(n * 2);
}