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Functions

Arrow functions

  • The full name is arrow function expressions, because arrow functions are always expressions.
  • Valid cases
    • Stored in a variable,
    • Passed as an argument to a function,
    • Stored in an object’s property.
  • Parameters
    • When there is only one parameter, the parentheses can be omitted.
    • When there is zero or multiple parameters, the parentheses cannot be omitted.
  • Function body
    • Concise body syntax: when there is only a single expression, curly braces, return, and semi-colons can all be omitted.
    • Block body syntax: when there is more than a single expression.
const fn0 = () => result;
const fn1 = _ => result;
const fn2 = singleParam => result;
const fn3 = (param1, param2) => result;
const fn4 = (param1, param2) => {
  return result;
};
  • When not to use arrow functions
    • Arrow functions are only expressions. There is no arrow function declaration.
    • There is a gotcha with the this keyword in arrow functions.

this and regular functions

Function Callthis
With the new operatorThe new object
With call / applySpecified by call / apply
Method of an objectThe object the method is called from
With no context- In non-strict mode: the global object
- In strict mode: undefined

this and arrow functions

  • this is based on the function’s surrounding context.
  • The value of this inside an arrow function is the same as the value of this outside the function.

Default function parameters

const fn1 = function (param1 = 'defaultValue1', param2 = 'defaultValue2') {
};

Defaults and destructuring arrays

function createGrid([width = 5, height = 5]) {
  return `Generates a ${width} x ${height} grid`;
}
 
createGrid([]);             // Generates a 5 x 5 grid
createGrid([2]);            // Generates a 2 x 5 grid
createGrid([2, 3]);         // Generates a 2 x 3 grid
createGrid([undefined, 3]); // Generates a 5 x 3 grid
  • The createGrid() function expects an array to be passed to it.
  • It uses destructuring to set the first and second items in the array.
  • If the array is empty or if it has only one item in it, then the default parameters kick in.
createGrid();               // throws an error
 
function createGrid([width = 5, height = 5] = []) {
  return `Generates a ${width} x ${height} grid`;
}
 
createGrid();               // Generates a 5 x 5 grid
  • The function expects an array to be passed in for destructuring.
    • Pass in non-array arguments will result in error.
    • Pass in an array, and then other arguments is fine.
  • When no parameter is passed in, it throws an error.
  • With = [], if createGrid() is called without any argument, it will use this default empty array.

Defaults and destructuring objects

  • A function can have an object be a default parameter and use object destructuring.
  • An advantage of object defaults over array defaults is that it is easier to skip a parameter.
function createSundae({scoops = 1, toppings = ['Hot Fudge']}) {
  const scoopText = scoops === 1 ? 'scoop' : 'scoops';
  return `Your sundae has ${scoops} ${scoopText} with ${toppings.join(' and ')} toppings.`;
}
 
createSundae({});
// Your sundae has 1 scoop with Hot Fudge toppings.
createSundae({scoops: 2});
// Your sundae has 2 scoops with Hot Fudge toppings.
createSundae({scoops: 2, toppings: ['Sprinkles']});
// Your sundae has 2 scoops with Sprinkles toppings.
createSundae({toppings: ['Cookie Dough']});
// Your sundae has 1 scoop with Cookie Dough toppings.
  • The same issue applies to calling createSundae without any argument.
  • Use {scoops = 1, toppings = ['Hot Fudge']} = {} to specify a default object value.

JavaScript class

  • JavaScript classes are just a thin mirage over regular functions and prototypal inheritance.
    • The constructor function is called with the new keyword.
    • The constructor function, by convention, starts with a capital letter.
    • The constructor function controls the setting of data on the objects that will be created.
    • “Inherited” methods are placed on the constructor function’s prototype object.

ES5 classes

function Plane(numEngines) {
  this.numEngines = numEngines;
  this.enginesActive = false;
}
 
Plane.prototype.startEngines = function () {
  console.log('starting engines...');
  this.enginesActive = true;
};

ES6 classes

  • ES6 provides a new class syntax.
  • A class is just a function.
  • When creating a new instance, the new keyword must be used. Otherwise, an error will be thrown.
  • Use keyword static to mark a method as static.
class Plane {
  // everything inside the constructor function
  // is inside the new constructor method
  constructor(numEngines) {
    this.numEngines = numEngines;
    this.enginesActive = false;
  }
 
  // methods are placed inside the class
  startEngines() {
    console.log('starting engines…');
    this.enginesActive = true;
  }
}
 
typeof Plane; // function
  • Benefits of classes
    • Less setup
    • Clearly defined constructor function
    • Everything is contained

ES6 subclasses

  • Use super and extends to extend a class.
  • In the constructor function, super must be called before this can be used.
// ES6 version
 
class Tree {
  constructor(size = '10', leaves = {spring: 'green', summer: 'green', fall: 'orange', winter: null}) {
    this.size = size;
    this.leaves = leaves;
    this.leafColor = null;
  }
 
  changeSeason(season) {
    this.leafColor = this.leaves[season];
    if (season === 'spring') {
      this.size += 1;
    }
  }
}
 
class Maple extends Tree {
  constructor(syrupQty = 15, size, leaves) {
    // super as a function
    super(size, leaves);
    this.syrupQty = syrupQty;
  }
 
  changeSeason(season) {
    // super as an object
    super.changeSeason(season);
    if (season === 'spring') {
      this.syrupQty += 1;
    }
  }
 
  gatherSyrup() {
    this.syrupQty -= 3;
  }
}
 
// ES5 version
 
function Tree(size, leaves) {
  this.size = (typeof size === "undefined")? 10 : size;
  const defaultLeaves = {spring: 'green', summer: 'green', fall: 'orange', winter: null};
  this.leaves = (typeof leaves === "undefined")?  defaultLeaves : leaves;
  this.leafColor;
}
 
Tree.prototype.changeSeason = function(season) {
  this.leafColor = this.leaves[season];
  if (season === 'spring') {
    this.size += 1;
  }
}
 
function Maple (syrupQty, size, leaves) {
  Tree.call(this, size, leaves);
  this.syrupQty = (typeof syrupQty === "undefined")? 15 : syrupQty;
}
 
Maple.prototype = Object.create(Tree.prototype);
// maple's prototype has been overwritten, so
// the constructor property and the original
// constructor function needs to be reconnected
Maple.prototype.constructor = Maple;
 
Maple.prototype.changeSeason = function(season) {
  Tree.prototype.changeSeason.call(this, season);
  if (season === 'spring') {
    this.syrupQty += 1;
  }
}
 
Maple.prototype.gatherSyrup = function() {
  this.syrupQty -= 3;
}
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