ES2017.MD

ES2017 (ES8)

See the ES2017 standard for full specification of the ECMAScript 8 language.

ES8 includes the following new features:

• Async functions
• Object.entries
• Object.values
• Object.getOwnPropertyDescriptors
• Trailing commas

---

async function

The asynchronous function returns an AsyncFunction object and operates asynchronously via the event loop. The syntax is very similar to synchronous functions

Syntax:

• async function name([param[, param[, ... param]]]) { statements }
• @name: the function name
• @param: param of the function
• statements: body of the function
• returns: Promise

const resolveAfter3Seconds = function() {
  console.log('starting 3 second promsise')
  return new Promise(resolve => {
    setTimeout(function() {
      resolve(3)
      console.log('done in 3 seconds')  
    }, 3000)  
  })  
}

const resolveAfter1Second = function() {
  console.log('starting 1 second promise')
  return new Promise(resolve => {
      setTimeout(function() {
        resolve(1) 
        console.log('done, in 1 second') 
      }, 1000)
  })  
}

const sequentialStart = async function() {
  console.log('***SEQUENTIAL START***')
  const one = await resolveAfter1Second()
  const three = await resolveAfter3Seconds()

  console.log(one)
  console.log(three)
}

sequentialStart() // invoke async function

Object.entries()

Object.entries gives us the ability to get an object's enurmerable property pairs by returning an array of any given object's own eumberable properties. /ie: [key, value] pairs. Note that the order is the same as provided by the for...in loop.

Syntax:

• Object.entries(obj)
• @params: obj
• returns: Array

Examples:

basic example

const obj = { self: 'that', norf: 'quux' }
console.log(Object.entries(obj))

// => [ ['self', 'that'], ['norf', 'quux'] ]

array like object with random key ordering

const obj = { 50: 'a', 1: 'b', 5: 'c' }
console.log(Object.entries(obj)) 

// => [ ['1', 'b'], ['5', 'c'], ['50', 'a'] ]

Object.values()

Object.values lets us return an array of a given object's own enumerable property values. Note that the order is the same as provided by the for...in loop.

Syntax:

• Object.values(obj)
• @params: obj
• returns: Array

Examples:

basic example

const obj = { a: 100, b: 200 }
console.log(Object.values(obj))

// => [100, 200]

mixed

const obj = { foo: 'foo', bar: [100, 200], baz: 55 }
console.log(Object.values(obj)) 

// => ['foo', [100, 200], 55 ]

string

const myStr = 'Lufthansa'
console.log(Object.values(myStr))

// => ["L", "u", "f", "t", "h", "a", "n", "s", "a"]

Object.getOwnPropertyDescriptors()

Object.getOwnPropertyDescriptors() returns all own property descriptors of a given object. A property descriptor is a record with one of the following attributes:

• value
• writable
• get
• set
• configurable
• enumerable

let myObj = {
  property1: 'foo',
  property2: 'bar',
  property3: 42,
  property4: () => console.log('prop4')  
}

Object.getOwnPropertyDescriptors(myObj)

/*
{ property1: {…}, property2: {…}, property3: {…}, property4: {…} }
  property1: {value: "foo", writable: true, enumerable: true, configurable: true}
  property2: {value: "bar", writable: true, enumerable: true, configurable: true}
  property3: {value: 42, writable: true, enumerable: true, configurable: true}
  property4: {value: ƒ, writable: true, enumerable: true, configurable: true}
  __proto__: Object
*/

Trailing Commas

Trailing commas are now allowed in JavaScript - they are now ignored

trailing commas in array

let arr = [
  10,
  20,
  30,
  40,
  50,  
]

arr // [10, 20, 30, 40,  50]
arr.length // 5 

trailing commas in object

const obj = {
  trailing: 'comma',
  is: 'allowed',
  in: 'JavaScript..', 
}