> For the complete documentation index, see [llms.txt](https://tkssharma.gitbook.io/react-training/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://tkssharma.gitbook.io/react-training/day-06/es2015-or-16-quick-recap.md).

# ES2015 | 16 Quick Recap

A quick overview of new JavaScript features in ES2015, ES2016, ES2017, ES2018 and beyond.

## Block scoping

**Let**

```
function fn () {
  let x = 0
  if (true) {
    let x = 1 // only inside this `if`
  }
}
 
 
```

**Const**

```
const a = 1
```

`let` is the new `var`. Constants work just like `let`, but can’t be reassigned. See: [Let and const](https://babeljs.io/learn-es2015/#let--const)

#### Backtick strings <a href="#backtick-strings" id="backtick-strings"></a>

**Interpolation**

```
const message = `Hello ${name}`
```

**Multiline strings**

```
const str = `
hello
world
`
```

Templates and multiline strings. See: [Template strings](https://babeljs.io/learn-es2015/#template-strings)

#### Binary and octal literals <a href="#binary-and-octal-literals" id="binary-and-octal-literals"></a>

```
let bin = 0b1010010
let oct = 0o755
```

See: [Binary and octal literals](https://babeljs.io/learn-es2015/#binary-and-octal-literals)

### New methods

**New string methods**

```
"hello".repeat(3)
"hello".includes("ll")
"hello".startsWith("he")
"hello".padStart(8) // "   hello"
"hello".padEnd(8) // "hello   " 
"hello".padEnd(8, '!') // hello!!!
"\u1E9B\u0323".normalize("NFC")
```

## Classes

```
class Circle extends Shape {
```

**Constructor**

```
  constructor (radius) {
    this.radius = radius
  }
 
```

**Methods**

```
  getArea () {
    return Math.PI * 2 * this.radius
  }
 
```

**Calling superclass methods**

```
  expand (n) {
    return super.expand(n) * Math.PI
  }
 
```

**Static methods**

```
  static createFromDiameter(diameter) {
    return new Circle(diameter / 2)
  }
}
 
```

Syntactic sugar for prototypes. See: [Classes](https://babeljs.io/learn-es2015/#classes)

## Promises

#### Making promises <a href="#making-promises" id="making-promises"></a>

```
new Promise((resolve, reject) => {
  if (ok) { resolve(result) }
  else { reject(error) }
})
 
```

For asynchronous programming. See: [Promises](https://babeljs.io/learn-es2015/#promises)

#### Using promises <a href="#using-promises" id="using-promises"></a>

```
promise
  .then((result) => { ··· })
  .catch((error) => { ··· })
 
 
```

#### Using promises with finally <a href="#using-promises-with-finally" id="using-promises-with-finally"></a>

```
promise
  .then((result) => { ··· })
  .catch((error) => { ··· })
  .finally(() => { // logic independent of success/error })
 
```

The handler is called when the promise is fulfilled or rejected.

#### Promise functions <a href="#promise-functions" id="promise-functions"></a>

```
Promise.all(···)
Promise.race(···)
Promise.reject(···)
Promise.resolve(···)
```

#### Async-await <a href="#async-await" id="async-await"></a>

```
async function run () {
  const user = await getUser()
  const tweets = await getTweets(user)
  return [user, tweets]
}
 
 
```

`async` functions are another way of using functions.

See: [async function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/async_function)

## Destructuring

#### De-structuring assignment <a href="#destructuring-assignment" id="destructuring-assignment"></a>

**Arrays**

```
const [first, last] = ['Nikola', 'Tesla']
 
```

**Objects**

```
let {title, author} = {
  title: 'The Silkworm',
  author: 'R. Galbraith'
}
 
```

Supports for matching arrays and objects. See: [Destructuring](https://babeljs.io/learn-es2015/#destructuring)

#### Default values <a href="#default-values" id="default-values"></a>

```
const scores = [22, 33]
const [math = 50, sci = 50, arts = 50] = scores
```

```
// Result:
// math === 22, sci === 33, arts === 50
```

Default values can be assigned while destructuring arrays or objects.

#### Function arguments <a href="#function-arguments" id="function-arguments"></a>

```
function greet({ name, greeting }) {
  console.log(`${greeting}, ${name}!`)
}
 
```

```
greet({ name: 'Larry', greeting: 'Ahoy' })
```

Destructuring of objects and arrays can be also be done in function arguments.

#### Default values <a href="#default-values-1" id="default-values-1"></a>

```
function greet({ name = 'Rauno' } = {}) {
  console.log(`Hi ${name}!`);
}
 
```

```
greet() // Hi Rauno!
greet({ name: 'Larry' }) // Hi Larry!
```

#### Reassigning keys <a href="#reassigning-keys" id="reassigning-keys"></a>

```
function printCoordinates({ left: x, top: y }) {
  console.log(`x: ${x}, y: ${y}`)
}
 
```

```
printCoordinates({ left: 25, top: 90 })
```

This example assigns `x` to the value of the `left`key.

#### Loops <a href="#loops" id="loops"></a>

```
for (let {title, artist} of songs) {
  ···
}
 
```

The assignment expressions work in loops, too.

#### Object de-structuring <a href="#object-destructuring" id="object-destructuring"></a>

```
const { id, ...detail } = song;
 
```

Extract some keys individually and remaining keys in the object using rest (…) operator

## Spread

#### Object spread <a href="#object-spread" id="object-spread"></a>

**with Object spread**

```
const options = {
  ...defaults,
  visible: true
}
 
```

**without Object spread**

```
const options = Object.assign(
  {}, defaults,
  { visible: true })
```

The Object spread operator lets you build new objects from other objects.

See: [Object spread](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_operator)

#### Array spread <a href="#array-spread" id="array-spread"></a>

**with Array spread**

```
const users = [
  ...admins,
  ...editors,
  'rstacruz'
]
 
 
```

**without Array spread**

```
const users = admins
  .concat(editors)
  .concat([ 'rstacruz' ])
```

The spread operator lets you build new arrays in the same way.

See: [Spread operator](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/Spread_operator)

## Functions

#### Function arguments <a href="#function-arguments-1" id="function-arguments-1"></a>

**Default arguments**

```
function greet (name = 'Jerry') {
  return `Hello ${name}`
}
 
```

**Rest arguments**

```
function fn(x, ...y) {
  // y is an Array
  return x * y.length
}
 
```

**Spread**

```
fn(...[1, 2, 3])
// same as fn(1, 2, 3)
 
```

Default, rest, spread. See: [Function arguments](https://babeljs.io/learn-es2015/#default--rest--spread)

**Fat arrows**

```
setTimeout(() => {
  ···
})
 
```

**With arguments**

```
readFile('text.txt', (err, data) => {
  ...
})
 
```

**Implicit return**

```
numbers.map(n => n * 2)
// No curly braces = implicit return
// Same as: numbers.map(function (n) { return n * 2 })
numbers.map(n => ({
  result: n * 2
})
// Implicitly returning objects requires parentheses around the object
 
 
 
 
```

Like functions but with `this` preserved. See: [Fat arrows](https://babeljs.io/learn-es2015/#arrows-and-lexical-this)

## Objects

#### Shorthand syntax <a href="#shorthand-syntax" id="shorthand-syntax"></a>

```
module.exports = { hello, bye }
// Same as: module.exports = { hello: hello, bye: bye }
```

See: [Object literal enhancements](https://babeljs.io/learn-es2015/#enhanced-object-literals)

#### Methods <a href="#methods-1" id="methods-1"></a>

```
const App = {
  start () {
    console.log('running')
  }
}
// Same as: App = { start: function () {···} }
 
```

See: [Object literal enhancements](https://babeljs.io/learn-es2015/#enhanced-object-literals)

#### Getters and setters <a href="#getters-and-setters" id="getters-and-setters"></a>

```
const App = {
  get closed () {
    return this.status === 'closed'
  },
  set closed (value) {
    this.status = value ? 'closed' : 'open'
  }
}
 
 
```

See: [Object literal enhancements](https://babeljs.io/learn-es2015/#enhanced-object-literals)

#### Computed property names <a href="#computed-property-names" id="computed-property-names"></a>

```
let event = 'click'
let handlers = {
  [`on${event}`]: true
}
// Same as: handlers = { 'onclick': true }
 
```

See: [Object literal enhancements](https://babeljs.io/learn-es2015/#enhanced-object-literals)

#### Extract values <a href="#extract-values" id="extract-values"></a>

```
const fatherJS = { age: 57, name: "Brendan Eich" }

Object.values(fatherJS)
// [57, "Brendan Eich"]
Object.entries(fatherJS)
// [["age", 57], ["name", "Brendan Eich"]]
```

## Modules

#### Imports <a href="#imports" id="imports"></a>

```
import 'helpers'
// aka: require('···')
```

```
import Express from 'express'
// aka: const Express = require('···').default || require('···')
```

```
import { indent } from 'helpers'
// aka: const indent = require('···').indent
```

```
import * as Helpers from 'helpers'
// aka: const Helpers = require('···')
```

```
import { indentSpaces as indent } from 'helpers'
// aka: const indent = require('···').indentSpaces
```

`import` is the new `require()`. See: [Module imports](https://babeljs.io/learn-es2015/#modules)

#### Exports <a href="#exports" id="exports"></a>

```
export default function () { ··· }
// aka: module.exports.default = ···
```

```
export function mymethod () { ··· }
// aka: module.exports.mymethod = ···
```

```
export const pi = 3.14159
// aka: module.exports.pi = ···
```

`export` is the new `module.exports`. See: [Module exports](https://babeljs.io/learn-es2015/#modules)

## Generators

#### Generators <a href="#generators-1" id="generators-1"></a>

```
function* idMaker () {
  let id = 0
  while (true) { yield id++ }
}
```

```
let gen = idMaker()
gen.next().value  // → 0
gen.next().value  // → 1
gen.next().value  // → 2
```

It’s complicated. See: [Generators](https://babeljs.io/learn-es2015/#generators)

#### For..of iteration <a href="#forof-iteration" id="forof-iteration"></a>

```
for (let i of iterable) {
  ···
}
```

For iterating through generators and arrays. See: [For..of iteration](https://babeljs.io/learn-es2015/#iterators--forof)