What are the new things introduced in it?

ES6 introduced the concept of a rest element when working with array destructuring:

const numbers = [1, 2, 3, 4, 5]
[first, second, ...others] = numbers


const numbers = [1, 2, 3, 4, 5]
const sum = (a, b, c, d, e) => a + b + c + d + e
const sumOfNumbers = sum(...numbers)


ES2018 introduces the same but for objects.

Rest properties:

const { first, second, ...others } = { first: 1, second: 2, third: 3, fourth: 4, fifth: 5 }

first // 1
second // 2
others // { third: 3, fourth: 4, fifth: 5 }


Spread properties allow to create a new object by combining the properties of the object passed after the spread operator:

const items = { first, second, ...others }
items //{ first: 1, second: 2, third: 3, fourth: 4, fifth: 5 }


## Asynchronous iteration

The new construct for-await-of allows you to use an async iterable object as the loop iteration:

for await (const line of readLines(filePath)) {
console.log(line)
}


Since this uses await, you can use it only inside async functions, like a normal await (see async/await)

## Promise.prototype.finally()

When a promise is fulfilled, successfully it calls the then() methods, one after another.

If something fails during this, the then() methods are jumped and the catch() method is executed.

finally() allow you to run some code regardless of the successful or not successful execution of the promise:

fetch('file.json')
.then(data => data.json())
.catch(error => console.error(error))
.finally(() => console.log('finished'))


## Regular Expression improvements

### RegExp lookbehind assertions: match a string depending on what precedes it

This is a lookahead: you use ?= to match a string that’s followed by a specific substring:

/Roger(?=Waters)/

/Roger(?= Waters)/.test('Roger is my dog') //false
/Roger(?= Waters)/.test('Roger is my dog and Roger Waters is a famous musician') //true


?! performs the inverse operation, matching if a string is not followed by a specific substring:

/Roger(?!Waters)/

/Roger(?! Waters)/.test('Roger is my dog') //true
/Roger(?! Waters)/.test('Roger Waters is a famous musician') //false


Lookaheads use the ?= symbol. They were already available.

Lookbehinds, a new feature, uses ?<=.

/(?<=Roger) Waters/

/(?<=Roger) Waters/.test('Pink Waters is my dog') //false
/(?<=Roger) Waters/.test('Roger is my dog and Roger Waters is a famous musician') //true


A lookbehind is negated using ?<!:

/(?<!Roger) Waters/

/(?<!Roger) Waters/.test('Pink Waters is my dog') //true
/(?<!Roger) Waters/.test('Roger is my dog and Roger Waters is a famous musician') //false


### Unicode property escapes \p{…} and \P{…}

In a regular expression pattern you can use \d to match any digit, \s to match any character that’s not a white space, \w to match any alphanumeric character, and so on.

This new feature extends this concept to all Unicode characters introducing \p{} and its negation \P{}.

Any unicode character has a set of properties. For example Script determines the language family, ASCII is a boolean that’s true for ASCII characters, and so on. You can put this property in the graph parentheses, and the regex will check for that to be true:

/^\p{ASCII}+$/u.test('abc') //✅ /^\p{ASCII}+$/u.test('ABC@')  //✅
/^\p{ASCII}+$/u.test('ABC🙃') //❌  ASCII_Hex_Digit is another boolean property, that checks if the string only contains valid hexadecimal digits: /^\p{ASCII_Hex_Digit}+$/u.test('0123456789ABCDEF') //✅
/^\p{ASCII_Hex_Digit}+$/u.test('h') //❌  There are many other boolean properties, which you just check by adding their name in the graph parentheses, including Uppercase, Lowercase, White_Space, Alphabetic, Emoji and more: /^\p{Lowercase}$/u.test('h') //✅
/^\p{Uppercase}$/u.test('H') //✅ /^\p{Emoji}+$/u.test('H')   //❌
/^\p{Emoji}+$/u.test('🙃🙃') //✅  In addition to those binary properties, you can check any of the unicode character properties to match a specific value. In this example, I check if the string is written in the greek or latin alphabet: /^\p{Script=Greek}+$/u.test('ελληνικά') //✅
/^\p{Script=Latin}+\$/u.test('hey') //✅


Read more about all the properties you can use directly on the proposal.

### Named capturing groups

In ES2018 a capturing group can be assigned to a name, rather than just being assigned a slot in the result array:

const re = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/
const result = re.exec('2015-01-02')

// result.groups.year === '2015';
// result.groups.month === '01';
// result.groups.day === '02';


### The s flag for regular expressions

The s flag, short for single line, causes the . to match new line characters as well. Without it, the dot matches regular characters but not the new line:

/hi.welcome/.test('hi\nwelcome') // false
/hi.welcome/s.test('hi\nwelcome') // true