Flash cards
Review the key moves
What is the main idea behind Javascript 2015 (ES6)?
Lesson checks
Practice each idea before moving on
Short Mimo-style checks built from this lesson's code, terms, and sequence.
Which statement best captures the main point of this lesson?
Complete the missing token from the example code.
// ___ x is 10 {Put the learning moves in the order that makes the concept easiest to apply.
ECMAScript 2015
The second major revision to JavaScript .
ECMAScript 2015 is also known as ES6.
New Features in JavaScript 2015 (ES6)
| Feature | Description |
|---|---|
| The let keyword | Declares a variable with block scope |
| The const keyword | Declare a contant immutable variable |
| Arrow Functions | Allows short syntax for writing function expressions |
| {a,b} = Operator | Assigns object properties to variables (object destructuring) |
| [a,b] = Operator | Assigns array values to variables (array destructuring) |
| ... Operator | Spreads an array or iterable into individual elements |
| For/of | Loops through the values of iterable objects |
| Map Objects | Object with key-value pairs, similar but different from objects |
| Set Objects | Array that stores unique values |
| Classes | Templates for JavaScript Objects |
| Promises | Object representing the completion of an asynchronous operation |
| Symbol | A unique "hidden" identifier that no code can access |
| Default Parameters | Allows default values for function parameters |
| Rest Parameters | Allows functions to treat an indefinite number of arguments |
| String.includes() | Returns true if a string contains a specified value |
| String.startsWith() | Returns true if a string begins with a specified value |
| String.endsWith() | Returns true if a string ends with a specified value |
| Array entries() | Returns an iterator key/value pairs from an array |
| Array.from() | Creates an array from a string |
| Array keys() | Returns an iterator with the keys of an array |
| Array find() | Returns the value of the first element that passes a test |
| Array findIndex() | Returns the index of the first element that passes a test |
| Object.assign() | Copies properties from a source object to a target object |
| RegExp /u | Enables full Unicode support in a regular expression |
| RegExp /y | Performs a "sticky" search from the lastIndex property |
| isFinite() | returns true if the argument is not Infinity or NaN |
| IsNaN() | Returns true if the argument is NaN |
| Modules | Allows for breaking up your code into separate files |
| Reflect | Object for low-lewel operation on objects |
| Proxy | Object that wraps other objects for control operations |
Math Features
| Feature | Description |
|---|---|
| Math.trunc(x) | Returns the integer part of x |
| Math.sign(x) | Returns -1, 0 or 1 (x is negative, null or positive) |
| Math.cbrt(x) | Returns the cube root of x |
| Math.log2(x) | Returns the base 2 logarithm of x |
| Math.log10(x) | Returns the base 10 logarithm of x |
Number Features
| Feature | Description |
|---|---|
| Number.EPSILON | The difference between 1 and the smallest number greater than 1 |
| Number.MIN_SAFE_INTEGER | Minimum value that can be precisely represented |
| Number.MAX_SAFE_INTEGER | Maximum value that can be precisely represented |
| Number.isInteger() | Returns true if the argument is an integer |
| Number.isSafeInteger() | Returns true if the argument is a safe integer |
Browser Support
JavaScript 2015 is supported in all modern browsers since June 2017 :
| Chrome 51 | Edge 15 | Firefox 54 | Safari 10 | Opera 38 |
|---|---|---|---|---|
| May 2016 | Apr 2017 | Jun 2017 | Sep 2016 | Jun 2016 |
ES6 is not supported in Internet Explorer.
JavaScript let
The let keyword allows you to declare a variable with block scope.
Example
var x = 10;
// Here x is 10 {
let x = 2;
// Here x is 2
}
// Here x is 10Read more about let in the chapter: JavaScript Let .
JavaScript const
The const keyword allows you to declare a constant (a JavaScript variable with a constant value).
Constants are similar to let variables, except that the value cannot be changed.
Example
var x = 10;
// Here x is 10 {
const x = 2;
// Here x is 2
}
// Here x is 10Read more about const in the chapter: JavaScript Const .
Arrow Functions
Arrow functions is a short syntax for writing function expressions .
You don't need the function keyword, the return keyword, or the curly brackets .
Before Arrow:
let myFunction = function(a, b) {return a * b}With Arrow
let myFunction = (a, b) => a * b;Arrow functions do not have their own this . They are not well suited for defining object methods .
Arrow functions are not hoisted. They must be defined before they are used.
You can only omit the return keyword and the curly brackets if the function is a single statement. Because of this, it might be a good habit to always keep them:
Example
// This will not work
let myFunction = (x, y) => { x * y } ;
// This will not work
let myFunction = (x, y) => return x * y ;
// Only this will work
let myFunction = (x, y) => { return x * y };Learn more about Arrow Functions in the chapter: JavaScript Arrow Function .
Object Destructuring
Destructuring assignment makes it easy to assign array values and object properties to variables.
Example
// Create an Object
const person = {
firstName: "John", lastName: "Doe", age: 50, eyeColor: "blue"
};
// Destructuring Assignment
let { firstName, age } = person;When destructuring an object, you must use the same name for the variables as the corresponding object keys (names).
The order of the keys (names) does not matter.
Array Destructuring
Destructuring assignment makes it easy to assign array values and object properties to variables.
Example
// Create an Array
const fruits = ["Banana", "Orange", "Apple", "Mango"];
// Destructuring Assignment
let [fruit1, fruit2] = fruits;The Spread (...) Operator
......Example
const q1 = ["Jan", "Feb", "Mar"];
const q2 = ["Apr", "May", "Jun"];
const q3 = ["Jul", "Aug", "Sep"];
const q4 = ["Oct", "Nov", "Dec"];
const year = [...q1, ...q2, ...q3, ...q4];The For/Of Loop
The JavaScript for/of statement loops through the values of iterable objects.
for/of lets you loop over data structures that are iterable such as Arrays, Strings, Maps, NodeLists, and more.
The for/of loop has the following syntax:
for (
variable
of
iterable
) {
//
code block to be executed
}variable - For every iteration the value of the next property is assigned to the variable. Variable can be declared with const , let , or var .
iterable - An object that has iterable properties.
Looping over an Array
Example
const cars = ["BMW", "Volvo", "Mini"];
let text = "";
for (let x of cars) {
text += x + " ";
}Looping over a String
Example
let language = "JavaScript";
let text = "";
for (let x of language) {
text += x + " ";
}Learn more in the chapter: JavaScript Loop For/In/Of .
JavaScript Maps
A Map is an object that stores key-value pairs, similar to objects, but with differences:
- Keys can be of any data type (objects, functions, primitive values), unlike plain objects where keys are strings.
- Maintains the original insertion order of keys.
Being able to use an Object as a key is an important Map feature.
Example
const fruits = new Map([ ["apples", 500], ["bananas", 300], ["oranges", 200]
]);Learn more about Map objects, and the difference between a Map and an Array, in the the chapter: JavaScript Maps .
JavaScript Sets
A Set is an object that stores unique values of any type (primitive values, functions, objects).
A Set can only contain unique values. An attempt to add a duplicate value will be ignored.
Example
// Create a Set
const letters = new Set();
// Add some values to the Set letters.add("a"); letters.add("b"); letters.add("c");Learn more about Set objects in the the chapter: JavaScript Sets .
JavaScript Classes
JavaScript Classes are templates for JavaScript Objects.
Use the keyword class to create a class.
Syntax
class ClassName {
constructor() { ... }
}Example
class Car {
constructor(name, year) {
this.name = name;
this.year = year;
}
}The example above creates a class named "Car".
The class has two initial properties: "name" and "year".
A JavaScript class is not an object.
It is a template for JavaScript objects.
Using a Class
When you have a class, you can use the class to create objects:
Example
const myCar1 = new Car("Ford", 2014);
const myCar2 = new Car("Audi", 2019);Learn more about classes in the the chapter: JavaScript Classes .
JavaScript Promises
A JavaScript Promise is an object representing the completion or failure of an asynchronous operation and its values.
It is a placeholder for a value that may not yet be available, providing a structured way to handle asynchronous code.
Promise Syntax
const myPromise = new Promise(function(myResolve, myReject) {
// "Producing Code" (May take some time)
myResolve(); // when successful
myReject(); // when error
});
// "Consuming Code" (Must wait for a fulfilled Promise).
myPromise.then(
function(value) { /* code if successful */ },
function(error) { /* code if some error */ }
);Example Using a Promise
const myPromise = new Promise(function(myResolve, myReject) {
setTimeout(function() { myResolve("I love You !!"); }, 3000);
});
myPromise.then(function(value) {
document.getElementById("demo").innerHTML = value;
});Learn more about Promises in the the chapter: JavaScript Promises .
The Symbol Type
A JavaScript Symbol is a primitive data type just like Number, String, or Boolean.
It represents a unique "hidden" identifier that no other code can accidentally access.
For instance, if different coders want to add a person.id property to a person object belonging to a third-party code, they could mix each others values.
Using Symbol() to create a unique identifiers, solves this problem:
Example
const person = {
firstName: "John", lastName: "Doe", age: 50, eyeColor: "blue"
};
let id = Symbol('id');
person[id] = 140353;
// Now person[id] = 140353 // but person.id is still undefinedSymbols are always unique.
If you create two symbols with the same description they will have different values:
Symbol("id") == Symbol("id"); // falseDefault Parameter Values
ES6 allows function parameters to have default values.
Example
function myFunction(x, y = 10) {
// y is 10 if not passed or undefined
return x + y;
}
myFunction(5); // will return 15Function Rest Parameter
The rest parameter (...) allows a function to treat an indefinite number of arguments as an array:
Example
function sum(...args) {
let sum = 0;
for (let arg of args) sum += arg;
return sum;
}
let x = sum(4, 9, 16, 25, 29, 100, 66, 77);String.includes()
The includes() method returns true if a string contains a specified value, otherwise false :
Example
let text = "Hello world, welcome to the universe.";
text.includes("world") // Returns true