#es6 #assembler-school #master-in-software-engineering

Assembler School: ES6 Workshop

In this workshop you will learn the main features of ES6 and beyond.

Getting Started
Workshop Material
var vs let vs const
Block Scopes
Default Function Parameters
Arrow Functions
Destructuring
New Array Methods
ES Modules
Promises
fetch
async/await
Template Literals
Classes

Getting Started

First, you will need to clone the repo:

$ git clone https://github.com/assembler-school/es6-workshop.git

Then, you will have to install all the dependencies with npm:

$ npm install

Workshop Material

Slides

var vs let vs const

First, let's start with the differences between var, let and const.

One of the major changes introduced in ES6 is how we can declare variables.

In ES5 and before, we could only declare variables with the var keyword.

In ES6 we have two new ways to declare variables using let and const which were introduced to solve some of the issues we had when working with variables declared with the var keyword.

`var`

Hoisting

In JavaScript, variables declared with the var keyword hoist. This means that we can use them before we declared them without the runtime throwing an exception. If we access the variable before it is initialized, we can read its value, which will be undefined even if we have assigned a value to it.

// => "Alex"
console.log(myName);

var myName = "Alex";

Auto-globals

In JavaScript, if we don’t explicitly declare a variable we use in the program, the variable will be automatically created as a global variable ⚠️.

This can be solved by turning on the strict mode of executing JavaScript or by simply declaring all the variable we use ✅.

// This automatically creates a global variable
// with the name `myAge` and the value of 30
//
// This is considered to be a very bad practice
// You should avoid writing this type of code
myAge = 30;

console.log(myAge); // 30

By turning on strict mode we can run catch these kind of issues because the language will throw an error.

"use strict";

// ReferenceError: myAge is not defined
myAge = 30;

console.log(myAge); // 30

`let & const`

Hoisting

Variables that are declared using the let or const keywords do not hoist, which means that we cannot use them before we define them.

// console.log(jobTitle);
//             ^
// ReferenceError: Cannot access 'jobTitle' before initialization
console.log(jobTitle);

let jobTitle = "Developer";

TDZ (Temporal Dead-Zone)

Variables that are declared using the let or const keywords cannot be accessed before the line of code where we initialize them.

Before this line, let and const variables are in a TDZ (Temporal Dead Zone) in which we cannot read or change the contents of the variables.

// Start of our program
//
// Start of the TDZ
// console.log(jobTitle);
//             ^
// ReferenceError: Cannot access 'jobTitle' before initialization
console.log(jobTitle);

let jobTitle = "Developer"; // End of the TDZ

Variables that are declared using the var keyword are initialized with a default value of undefined if we access them before the variable is initialized.

On the other hand, variables declared with let or const are not initialized with any value and trying to read or change their values will cause an error.

console.log(myName); // undefined

var myName = "Alex";

// Start of the TDZ

// Uncaught ReferenceError:
// Cannot access 'jobTitle' before initialization

console.log(jobTitle); // uninitialized

let jobTitle = "Developer"; // End of the TDZ

Why aren’t let variables initialized with undefined?

Variables declared with let are not initialized with a value because the language also introduced the const keyword.

If a variable declared with const were initialized with a value of undefined, it would have two different values which would go against the theoretical meaning of a constant variable. It would first have a value of undefined and then it would have the value we assigned to it.

Since both let and const were introduced at the same time, the creators of the JavaScript language specified that both let and const should be in a TDZ until the variables are initialized. Therefore, let and const variables are not initialized.

`var, let & const`

var vs let

Variables that are declared using the var keyword can be redeclared as many times as we want and still be the same variable. On the other hand, variables created with let cannot be redeclared.

var myName = "Ana";
console.log(myName); // Ana

// `myName` is a label to the same variable
var myName = "Hello I am Ana";
console.log(myName); // => Hello I am Ana

let myAge = 30;
console.log(myAge); // => 30

let myAge = 40;
// Output:
// let myAge = 40;
//     ^
// SyntaxError: Identifier 'myAge' has already been declared

Block Scopes

`var`

Block Scope

Variables that are declared using the var keyword have a global or a function scope. Variables declared with let or const create a block scope besides the function scope that var also create.

function sayVarName() {
  if (true) {
    // var is only function scoped
    var myName = "Alex";
  }

  // The name variable is available in the entire scope of the function
  console.log(myName); // Alex
}

function sayLetName() {
  if (true) {
    // let is block scoped
    let myName = "Alex";
  }

  // ReferenceError: myName is not defined
  console.log(myName);
}

`let` & `const`

Block Scopes

A block scope is created when we use a let or const variable inside a function, a if/else, if/else if/else statement, a switch statement, try/catch, etc.

// global scope and block
let count = 1;
console.log(count); // => 1

// declares a block inside the function
function myName() {
  let count = 2;
  console.log(count); // => 2

  // declares a block inside the if statement
  if (true) {
    let count = 3;
    console.log(count); // => 3
  }
}

Other ways of creating a block scope:

{
  let firstName = "Dani";

  console.log(`My name is: ${firstName}`); // => My name is: Dani
}

console.log(`I am: ${firstName}`); // => ReferenceError: firstName is not defined

`var` vs `let` vs `const` key points

Default Function Parameters

Pre ES6, setting a default value to one of the parameters of a function was pretty verbose and long to write.

function compute(num, times) {
  if (num === undefined) {
    num = 1;
  }
  if (times === undefined) {
    times = 1;
  }

  return num * times;
}

let num1 = compute();
console.log(num1); // => 1

let num2 = compute(2);
console.log(num2); // => 2

let num3 = compute(2, 2);
console.log(num3); // => 4

In ES6, now it is much easier to set a default value to the parameters of a function if we don’t provide an argument for each parameter.

// Now it is much easier to set a default value
function compute(num = 1, times = 1) {
  return num * times;
}

let num1 = compute();
console.log(num1); // => 1

let num2 = compute(2);
console.log(num2); // => 2

let num3 = compute(2, 2);
console.log(num3); // => 4

Passing undefined as an argument will trigger the parameter to take the default value.

function compute(num = 1, times = 1) {
  return num * times;
}

let num1 = compute(undefined, 20);
console.log(num1); // => 20

let num2 = compute(undefined, undefined);
console.log(num2); // => 1

Arrow Functions

Pre ES6, we could define functions using the function keyword. Functions could be named or anonymous and they could be stored as the value of a variable.

// function declaration
function sum(a, b, c) {
  return a + b + c;
}

// named function expression
var sum = function sum(a, b, c) {
  return a + b + c;
};

// anonymous function expression
var sum = function (a, b, c) {
  return a + b + c;
};

In ES6, we can use a new syntax of creating function with arrow functions.

// ES5
var sum = function sum(a, b, c) {
  return a + b + c;
};

// ES6
// Can be written with an anonymous function as:
// Explicit function body and return statement
const sum = (a, b, c) => {
  return a + b + c;
};

// ES5
var sum = function (a, b, c) {
  return a + b + c;
};

// ES6
// Can be written with an anonymous function as:
// Implicit return statement
const sum = (a, b, c) => a + b + c;

Things to keep in mind when using arrow functions

Arrow functions are always anonymous

const arr = ["Monday", "Tuesday", "Wednesday"];

// The callback function is anonymous, if an error occurs inside it,
// it will be more difficult to debug because it will not have
// a name in the stack trace of the error
arr.map((day) => {
  console.log(day);

  if (day === "Wednesday") {
    throw new Error("BAD DAY");
    // Error: BAD DAY
    //     at /es6-workshop/src/main.js:10:11
  }
});

If we assign a name to the callback function we will be able to see the function name that caused the error:

const arr = ["Monday", "Tuesday", "Wednesday"];

arr.map(function logName(day) {
  console.log(day);

  if (day === "Wednesday") {
    throw new Error("BAD DAY");
    // Error: BAD DAY
    //     at logName (/es6-workshop/src/main.js:10:11)
  }
});

Different Ways To Handle Parameters In Arrow Functions

Anonymous functions can be written in different ways depending on the number of parameters of the function.

const fn1 = () => {}; // no parameter
const fn2 = (x) => {}; // one parameter, an identifier
const fn3 = (x, y) => {}; // several parameters

Different Ways To Specify The Function Body

Anonymous functions can have an implicit return statement if the function body is a single line of code. If not, we need to specify the function body in {} and include an explicit return statement if we want to return a value from the function.

// block
const fn1 = (x) => {
  return x * x;
};

// expression, equivalent to previous line
const fn2 = (x) => x * x;

Destructuring

Destructuring is a convenient way of extracting multiple values from data stored in (possibly nested) objects and arrays.

It can be used in locations that receive data and the way to extract the values is specified via destructuring patterns.

Object Destructuring

Before ES6 we had to capture the values individually using dot notation and store them in variables.

With ES6 we can capture the values in an easier way using a destructuring pattern to save the values in variables.

const person = {
  firstName: "Jane",
  lastName: "Doe",
};

// ES5 Way
const firstName = person.firstName;
const lastName = person.lastName;

console.log(firstName); // Jane
console.log(lastName); // Doe

// ES6 Way
const { firstName, lastName } = person;

console.log(firstName); // Jane
console.log(lastName); // Doe

Default Values

If the property don’t exist on the object we can provide a default value which will be used when the property is undefined.

const person = { firstName: "Jane" };

// ES5 Way
const lastName = person.lastName || "Doe";
console.log(lastName); // Doe

// ES6 Way
const { firstName, lastName = "Doe" } = person;
console.log(lastName); // Doe

Nested Destructuring

We can also destructure nested objects and their properties, but we need to provide a fallback default object for when the nested object doesn’t exist.

const person = { firstName: "Jane" };

const {
  // We can also destructure both the object and its properties.
  // Destructure the entire nested object:
  address = {},
  // Or the properties it has:
  address: {
    street,
    // We need provide a fallback `{}`
    // if the object doesn’t exist in the source:
  } = {},
} = person;

console.log(address); // => {}
console.log(street); // => undefined

Otherwise we get a TypeError for trying to access a property on the undefined value.

const person = { firstName: "Jane" };

const {
  // If we don’t provide a fallback {} we will get an error.
  address: { street },
} = person;

console.log(street); // => TypeError: Cannot read property 'street' of undefined

Array Destructuring

Destructuring can also be used to capture data from arrays and, in the same way as with objects, we can provide default values.

// we can skip elements with an empty comma
// the `last` item has a default value
let [zero, , , third, , , last = "Empty"] = [
  "Monday",
  "Tuesday",
  "Wednesday",
  "Thursday",
  "Friday",
];

console.log(zero); // Monday
console.log(third); // Thursday
console.log(last); // Empty

...rest Operator

We can use the ...rest operator to capture individual values in variables and all the other variables in an array.

It can also be used in functions to capture the parameters.

const [x, ...y] = ["a", "b", "c"];
console.log(x); // x = 'a';
console.log(y); // y = ['b', 'c']

// also works with function parameters
function args(a, b, ...rest) {
  console.log(a); // 1
  console.log(b); // 2
  console.log(rest); // [3, 4, 5, 6, 7]
}

args(1, 2, 3, 4, 5, 6, 7);

Practical Uses Of Destructuring #1

// Destructure function parameters
function removeBreakpoint({ url, line, column }) {
  console.log(url); // => "the-url"
  console.log(line); // => 33
  console.log(column); // => 60
}

let options = {
  url: "the-url",
  line: 33,
  column: 60,
};

removeBreakpoint(options);

function returnMultipleValues() {
  return {
    foo: 1,
    bar: 2,
  };
}

// Destructure the returned value of a function
const { foo, bar } = returnMultipleValues();

console.log(foo); // => 1
console.log(bar); // => 2

Practical Uses Of Destructuring #2

function weekDays() {
  return ["Monday", "Tuesday", "Wednesday"];
}

// Destructure the returned value of a function
const [monday, ...otherDays] = weekDays();

console.log(monday); // => Monday
console.log(otherDays); // => ["Tuesday", "Wednesday"]

New Array Methods

Array.`from()`

The Array.from() static method creates a new, shallow-copied Array instance from an array-like or iterable object.

// Array from a String
const arr = Array.from("foo"); // => [ "f", "o", "o" ]

// Array from a Set
const set = new Set(["foo", "bar", "baz", "foo"]);
Array.from(set); // => [ "foo", "bar", "baz" ]

// Array from an Array-like object (arguments)
function createArray() {
  return Array.from(arguments);
}

createArray(1, 2, 3); // => [ 1, 2, 3 ]

Array.`flat()`

The Array.flat() method creates a new array with all sub-array elements concatenated into it recursively up to the specified depth.

const arr1 = [1, 2, [3, 4]];
arr1.flat(); // => [1, 2, 3, 4]

const arr2 = [1, 2, [3, 4, [5, 6]]];
arr2.flat(); // => [1, 2, 3, 4, [5, 6]]

const arr3 = [1, 2, [3, 4, [5, 6]]];
// Use a flattening depth of 2 for nested arrays
arr3.flat(2); // => [1, 2, 3, 4, 5, 6]

const arr4 = [1, 2, [3, 4, [5, 6, [7, 8, [9, 10]]]]];
// Use a flattening depth of Infinity for nested arrays
arr4.flat(Infinity); // => [1, 2, 3, 4, 5, 6, 7, 8, 9, 10]

Array.`includes()`

The Array.includes() method determines whether an array includes a certain value among its entries, returning true or false as appropriate.

const array1 = [1, 2, 3];
console.log(array1.includes(2)); // => true

const pets = ["cat", "dog", "bat"];
console.log(pets.includes("cat")); // => true

Array.`find()`

The Array.find() method returns the value of the first element in the provided array that satisfies the provided testing function.

Otherwise, it returns undefined, indicating that no element passed the test.

const array1 = [5, 12, 8, 130, 44];

// const found = arrar1.find(elem => eleme > 10);

const found = array1.find(function (element) {
  return element > 10;
});

console.log(found); // => 12

Array.`findIndex()`

The Array.findIndex() method returns the index of the first element in the array that satisfies the provided testing function.

Otherwise, it returns -1, indicating that no element passed the test.

const array1 = [5, 12, 8, 130, 44];

const found = array1.findIndex(function (element) {
  return element > 10;
});

console.log(found); // => 1

ES Modules

Modules allow us to split our code in different files so that we can better reuse, organize, modularize, and maintain our code.

Declaring And Exporting Modules

We can either declare several functions or variables that we export as named exports, or we can declare a default export that will be the default exported function or variable exported from the entire module.

// Open the utils.js file to practice
// src/es-modules/utils.js
// Named exports
export function add(a, b) {
  return a + b;
}

export function minus(a, b) {
  return a - b;
}

const BASE_VALUE = 20;

// Default export
export default BASE_VALUE;

Importing Modules

Once we have declared a set of modules, we can import both the named exports as the default exports.

// Open the imports.js file to practice
// es-modules/imports.js

// import a named export
import { add } from "./utils";

// import a default export
import BASE_VALUE from "./utils";

const result1 = add(1, BASE_VALUE);

// Result is: 21
console.log(`Result is: ${result1}`);

Default Exports

There are multiple ways that we can use to default export a module.

// es-modules/default-exports.js
// method 1
const names = ["Ana", "Alex", "Mark", "John"];
export default names;

// or with the second method
export default ["Ana", "Alex", "Mark", "John"];

// modules-main.js
// They would all be imported this way:
// We can specify any name that we would like to use for the module
import names from "./default-exports";

console.log(names); // => ["Ana", "Alex", "Mark", "John"]

Importing All Modules

We can also create a namespace for all the modules we have created so that we can import all of them under a base name that we can use to call them.

// es-modules/namespace.js
export function trimPassword(password) {
  return password.trim();
}

export function encryptPassword(encrypt, password) {
  return encrypt(password);
}

/**
 * Run the `npm run start` script and open in the browser
 * the following url: http://localhost:1234/
 *
 * Then, you should open the ES Modules link and the
 * browser console to you should see the output
 */
// modules-main.js
import * as utils from "./es-modules/namespace.js";

function encrypt(string = "") {
  return string.toUpperCase();
}

const encryptedPassword = utils.encryptPassword(encrypt, "my-password-1234");

console.log(encryptedPassword); // => MY-PASSWORD-1234

Renaming An Imported Module

We can also create a namespace for all the modules we have created so that we can import all of them under a base name that we can use to call them.

// namespace.js
export function trimPassword(password) {
  return password.trim();
}

// modules-main.js
import { trimPassword as shortenPassword } from "./es-modules/namespace";

const password = shortenPassword("  12345   ");
console.log(password); // 12345

Promises

Why do we need promises

Due to the nature of modern JavaScript applications that mostly rely on building their interfaces dynamically, we need a way to communicate with remote servers asynchronously without blocking the rendering of the browser so that the user can still use the application or browser while the information is returned from the server.

Handling Asynchronous Code Before ES6

Before we had promises in JavaScript the only way that we could execute asynchronous code was by using callbacks, setTimeout or setInterval. However, error handling and control flow of the execution of the code wasn’t very easy to handle and understand.

setTimeout(function performLongOperation() {
  // performing a long operation
  // that would block the UI...
}, 3000);

setInterval(function executeEverySecond() {
  // a function that runs every second
  // that shouldn’t block the UI...
}, 1000);

Callback Hell

Another problem of traditional JavaScript is that by using callbacks we could end up with what is commonly known as the callback hell…

This happens when we need to perform a network request that is based on the previous response.

// Each nested function depends on the result of the enclosing function.
// This creates a nested structure of function calls that depend on each other.
loginUser(function (user) {
  getUserDetails(user, function (userDetails) {
    getUserCart(userDetails, function (cart) {
      getCartItems(cart, function (cartItems) {
        getCartItemComments(cartItems[0], function (comments) {
          console.log(comments);
        });
      });
    });
  });
});

Refactoring to Promises

The previous code could be refactored to use promises which allows us to avoid having a callback hell, to have more readable code and to be able better handle errors.

loginUser()
  .then((user) => {
    return getUserDetails(user);
  })
  .then((userDetails) => {
    return getUserCart(userDetails);
  })
  .then((cart) => {
    return getCartItems(cart);
  })
  .then((cartItems) => {
    return getCartItemComments(cartItems[0]);
  })
  .then((comments) => {
    console.log(comments);
  })
  .catch((error) => {
    console.error(error);
  });

Creating Promises

To create a Promise, we simply call the Promise constructor and pass it two callback functions that we can execute to indicate that the Promise has been resolved or rejected with a value.

function task() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      resolve("Hello world");
    }, 500);
  });
}

const myPromise = task();
console.log(myPromise); // => Promise {<pending>}

myPromise.then((result) => {
  console.log(result); // => Hello world

  console.log(myPromise); // => Promise {<fulfilled>: "Hello world"}
});

Rejecting Promises

To reject a Promise, we just need to execute the reject callback function with a value. It is recommended that you reject the promise with an Error object so that it contains useful information about the error like the error message and stack trace of where the error has happened.

function task() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      reject(new Error("Something went wrong"));
    }, 500);
  });
}

const myPromise = task();

console.log(myPromise); // => Promise {<pending>}

myPromise.catch((error) => {
  // Something went wrong
  console.log(error.message);

  console.log(myPromise); // => Promise {<rejected>: Error: Something went wrong
});

Promise States

Promises can have 3 states.

pending: this is the base state of the promise before it is resolved or rejected.
fulfilled: this is the state that the promise has after it has been resolved with a value
rejected: this is the state that the promise has after it has been rejected with an error

fetch

The fetch() method is a browser API that can be used in all the browsers that support ES6 to perform network requests using promises.

It was designed to achieve the same functionality as using XMLHttpRequest but in a more powerful way.

// Open the src/node-fetch.js file to test it with node or with the browser
fetch("https://jsonplaceholder.typicode.com/users")
  .then(function (response) {
    return response.json();
  })
  .then(function (users) {
    console.log(users);
  });

Error Handling with `fetch()`

fetch() will only reject on network failure or if anything prevented the request from completing. These errors can be caught in the .catch() method.

fetch("https://jsonplaceholder.typicode.com/users/")
  .then(function (response) {
    // fetch will only reject on network failure
    // or if anything prevented the request from completing
    // For any other error it will set the response.ok
    // property to false
    if (!response.ok) {
      throw new Error(`Request failed with: ${response.status}`);
    }

    return response.json();
  })
  .then(function (users) {
    console.log(users);
  })
  .catch(function (error) {
    // we can handle network errors in the .catch() method
    console.log(error);
  });

Specifying Request Options

We can also specify the different options we might use to define the request such as the request method, among others.

If you'd like to learn more about fetch, you can use this link: Fetch API

fetch("https://jsonplaceholder.typicode.com/users/", {
  // GET, POST, PUT, DELETE, etc.
  method: "POST",
  headers: {
    "Content-Type": "application/json",
  },
  // body data type must match "Content-Type" header
  body: JSON.stringify({ firstName: "Dani" }),
}).then((response) => console.log(response.ok));

`async`/`await`

`async` Functions

async functions are a new feature of the language that allows us to work with promises in an easier way. The value returned by the async function will be wrapped in a resolved Promise.

// basic syntax of an `async` function
async function getUsers() {
  return [];
}

// same as
function getUsers() {
  return Promise.resolve([]);
}

`await` Keyword

The await keyword can only be used inside async functions and it allows us to pause the execution of the function until the value we are awaiting is resolved or rejected.

async function getUsers() {
  const response = await fetch("https://jsonplaceholder.typicode.com/users/");

  const users = await response.json();

  console.log(users); // [{...}, {...}, {...}]
}

`await`

The await keyword will wait until the promise is either rejected or resolved and it will either return the resolved value or it will throw the rejected error.

async function myFunction() {
  let promise = new Promise((resolve, _reject) => {
    setTimeout(() => resolve("done!"), 2000);
  });

  console.log("Before await"); // => Before await

  // wait until the promise resolves (*)
  let result = await promise;

  console.log("After await"); // => After await

  console.log(result); // => done!
}

You Cannot Use the `await` Keyword Outside an `async` Function

It is important to remember that we can only use the await keyword inside async functions, otherwise the language will throw an error.

// not an async function!!
function getUsers() {
   // ERROR: Can not use keyword 'await' outside an async function
   const response = await fetch("https://jsonplaceholder.typicode.com/users/");
   const users = await response.json();
   console.log(users); // [{...}, {...}, {...}]
}

`async`/`await` Error Handling

The best way to handle errors with async/await functions is to wrap the code in a try/catch block.

function task() {
  return new Promise((resolve, reject) => {
    setTimeout(() => {
      reject(new Error("Ups"));
    }, 1000);
  });
}

async function waitForTask() {
  let result = "";

  try {
    result = await task();
  } catch (error) {
    result = error.message;
  }

  console.log(result); // Ups
}

Template Literals

Template literals allow us to perform string concatenation in a much easier way than before.

const age = 20;
const name = "Ana";

// pre-ES6
var es5Ana = "My name is: " + name + " and I am " + age + " years old";

// ES6
const es6Ana = `My name is: ${name} and I am ${age} years old`;

Multi-Line Strings

Template literals also make it easier to create multi-line string.

console.log("string text line 1\n" + "string text line 2");
// "string text line 1
// string text line 2"

console.log(`string text line 1
string text line 2`);
// "string text line 1
// string text line 2"

Tagged Template Literals

Tagged template literals allow us to perform an operation on each of the strings and variables of the template literal received.

function tag(strings, expression1, expression2, expression3) {
  console.log(strings); // => ["My name is ", ", I’m a "]

  console.log(strings[0]); // => "My name is "
  console.log(strings[1]); // => ", I’m a "

  console.log(expression1); // => Ana
  console.log(expression2); // => 30
  console.log(expression3); // => developer
}

const name = "Ana";
const age = 30;
const profession = "developer";

tag`My name is ${name}, I’m a ${age} ${profession}`;

Classes

In ES6 JavaScript gained support for the class keyword, making it easier to program in a Object Oriented way.

class Animal {
  constructor(name, species, age) {
    this.name = name;
    this.species = species;
    this.age = age;
  }
}

const dog = new Animal("Spark", "dog", 5);

// Animal
// {
//   name: "Spark",
//   species: "dog",
//   age: 5
// }

console.log(dog);

Class methods

As in all other languages, classes can also have methods that we can call.

class Animal {
  constructor(name, species, age) {
    this.name = name;
    this.species = species;
    this.age = age;
  }

  saySpecies() {
    console.log(this.species);
  }

  sayName() {
    console.log(this.name);
  }
}

const dog = new Animal("Spark", "dog", 5);

dog.saySpecies(); // => dog
dog.sayName(); // => Spark

Static Methods

The static keyword defines a static method for a class. Static methods are called without instantiating their class and cannot be called through a class instance. Static methods are often used to create utility functions for an application.

class Point {
  constructor(x, y) {
    this.x = x;
    this.y = y;
  }

  static distance(a, b) {
    const dx = a.x - b.x;
    const dy = a.y - b.y;

    return Math.hypot(dx, dy);
  }
}

const p1 = new Point({ x: 5, y: 5 }, { x: 10, y: 15 });
console.log(p1.distance); // => undefined

console.log(Point.distance({ x: 5, y: 5 }, { x: 10, y: 15 })); // => 11.180339887498949

Sub Classing with `extends`

The extends keyword is used in class declarations or class expressions to create a class as a child of another class.

class Animal {
  constructor(name) {
    this.name = name;
  }
  speak() {
    console.log(`${this.name} makes a noise.`);
  }
}

class Dog extends Animal {
  constructor(name) {
    // call the super class constructor and pass in the name parameter
    super(name);
  }
  speak() {
    console.log(`${this.name} barks.`);
  }
}

let d = new Dog("Mitzie");
d.speak(); // => Mitzie barks.

Super `class` Calls with `super()`

The super keyword is used to call corresponding methods of the super class. This is one advantage over prototype-based inheritance.

class Cat {
  constructor(name) {
    this.name = name;
  }

  speak() {
    console.log(`${this.name} makes a noise.`);
  }
}

class Lion extends Cat {
  speak() {
    super.speak();
    console.log(`${this.name} roars.`);
  }
}

let l = new Lion("Fuzzy");
l.speak();
// Fuzzy makes a noise.
// Fuzzy roars.

Author

Dani Lucaci

License

MIT

Assembler School: ES6 Workshop

Table of Contents

Getting Started

Workshop Material

var vs let vs const

var

Hoisting

Auto-globals

let & const

Hoisting

TDZ (Temporal Dead-Zone)

Why aren’t let variables initialized with undefined?

var, let & const

var vs let

Block Scopes

var

Block Scope

let & const

Block Scopes

var vs let vs const key points

Default Function Parameters

Arrow Functions

Things to keep in mind when using arrow functions

Different Ways To Handle Parameters In Arrow Functions

Different Ways To Specify The Function Body

Destructuring

Object Destructuring

Default Values

Nested Destructuring

Array Destructuring

...rest Operator

Practical Uses Of Destructuring #1

Practical Uses Of Destructuring #2

New Array Methods

Array.from()

Array.flat()

Array.includes()

Array.find()

Array.findIndex()

ES Modules

Declaring And Exporting Modules

Importing Modules

Default Exports

Importing All Modules

Renaming An Imported Module

Promises

Why do we need promises

Handling Asynchronous Code Before ES6

Callback Hell

Refactoring to Promises

Creating Promises

Rejecting Promises

Promise States

fetch

Error Handling with fetch()

Specifying Request Options

async/await

async Functions

await Keyword

await

You Cannot Use the await Keyword Outside an async Function

async/await Error Handling

Template Literals

Multi-Line Strings

Tagged Template Literals

Classes

Class methods

Static Methods

Sub Classing with extends

Super class Calls with super()

Author

License

About

Languages

`var`

`let & const`

`var, let & const`

`var`

`let` & `const`

`var` vs `let` vs `const` key points

Array.`from()`

Array.`flat()`

Array.`includes()`

Array.`find()`

Array.`findIndex()`

Error Handling with `fetch()`

`async`/`await`

`async` Functions

`await` Keyword

`await`

You Cannot Use the `await` Keyword Outside an `async` Function

`async`/`await` Error Handling

Sub Classing with `extends`

Super `class` Calls with `super()`