INF310: Modern JS, Fall 2017 > Language Features > Functions
Anatomy of a function

Functions are objects that inherit from Function.prototype. To become a function, an object must implement the [[Call]] and [[Construct]] internal methods.


Functions have a declaration, also called a function signature and a body. The declaration contains an optional name and a list of parameters. The body is a code block (ordered statements).


All functions can return a value through the return statement. Arrow functions return the value of the last expression. If no explicit value is specified, the undefined value is returned by default.

Executing a function creates a new internal scope object.

  // the number of parameters the function formally accepts
Statement vs expression form

Both forms define a function value and bind it to a variable with the given name. But function statements are hoisted. The following two snippets of code are equivalent.

  // a function statement, the name is mandatory
  function identity(parameter) {
    return parameter;
  // a function expression
  var identity = function identity(parameter) {
    return parameter;
Arrow functions
  // single argument arrow function declaration
  // returns the value of the last (and only) expression
  const identity = id => id;

  // multiple parameters
  const add = (a, b) => a + b;

  // with a function body
  const validate = (input) => {
    if (input !== 'correct') {
      throw new Error('not correct');

A method is a function which is assigned as a property of an object.

  // es5
  var object = {
    method: function () { doStuff(); }

  // es6
  const object = {
    method() { doStuff(); }
Calling context

When using dot notation to invoke a method e.g. object.method() the value of this in the scope of the function execution is assigned the object value.

const getFullName = function fullName() {
  return `${this.firstName} ${this.lastName}`;

const person  = {
  firstName: 'John',
  lastName: 'Doe',
  getName: getFullName

person.getName(); // returns "John Doe"
Arrows are not methods

Arrow functions used as methods have their this assigned to the this value of the outer scope. Their this is NOT the object from their invocation.

// this = undefined
const getFullName = () => `${this.firstName} ${this.lastName}`;

const person  = {
  firstName: 'John',
  lastName: 'Doe',
  getName: getFullName

person.getName(); // throws an error
Lexical scopes

Functions have access to the scope containing their definition, and it's parent scopes.

function factory(factoryOptions) {
  const config = getConfig();

  return function instance(instanceOptions) {
    // this function can acces all of these:
Execution context

The apply and call methods on Function.prototype allow us to execute functions with explicitly set parameters and this value.

  function f(x) { return x * (this.y + this.z); }
  const useAsThis = { y:1, z: 2 };
  const parameter1 = 5;

   // executes the function with context as the "this" argument, parameter1, parameter2);

  // executes the function with context as the "this" argument
  f.apply(useAsThis, [parameter1, parameter2]);

This pattern is going out of style due to proper module semantics in the new versions of JavaScript

// the IEFE pattern:
// declare a function and execute it immediately
(function () {
  var data = getData();

This pattern is going out of style due to proper module semantics in the new versions of JavaScript

// this is an old module pattern:
// assign the result of an IEFE to a variable
// variables in the IEFE's scope are private to the module
var myModule = (function () {
  var privateVar = 'private';

  return {
    publicMethod() {
      return privateVar;

myModule.publicMethod(); // "private"
myModule.privateVar // undefined
Higher Order Functions

Higher order functions are the bread and butter of functional programming

  array.filter(element => isValid(element));

  setTimeout(function () {
    console.log("this is a callback");
  }, 1000);

  http.createServer((request, response) => {
    response.send('hello world!');
  // Currying
  function name(family) {
    return function (personal) {
      return `${personal} ${family}`;
  // with an arrow function
  const name = family => personal => `${personal} ${family}`;

  // usage
  const simpson = name('Simpson');
  const homer = simpson('Homer');
  const marge = simpson('Marge');
  const bart = name('Simpson')('Bart');
  function name(family, personal) {
    return `${personal} ${family}`;

  const magy = name('Simpson', 'Magy');

  // bind returns a new function
  // first argument will be used for "this"
  const simpson = func.bind(null, 'Simpson');

  const lisa = simpson('Lisa');