ES2018 is now finalized. Here's a summary of the most useful new features.

Promise.finally

Runs regardless of whether the Promise resolves or rejects:

fetch("/api/data")
  .then((res) => res.json())
  .catch((err) => console.error(err))
  .finally(() => {
    this.loading = false; // close loading whether success or failure
  });

Much more elegant than writing loading = false in both then and catch.

Object Rest/Spread (Finally Here)

// spread: copy and override properties
const user = { name: "Alice", age: 25, role: "admin" };
const updated = { ...user, age: 26 };

// rest: extract remaining properties
const { role, ...userWithoutRole } = user;
// userWithoutRole = { name: 'Alice', age: 25 }

Previously only arrays supported this; object spread officially entered the standard with ES2018.

Async Iteration (for await...of)

// Process multiple async operations in sequence
async function processItems(ids) {
  for await (const item of fetchItems(ids)) {
    await saveItem(item);
  }
}

// Custom async iterator
async function* generateSequence(start, end) {
  for (let i = start; i <= end; i++) {
    await delay(100);
    yield i;
  }
}

for await (const num of generateSequence(1, 5)) {
  console.log(num); // 1, 2, 3, 4, 5 (every 100ms)
}

Regex Improvements

// Named capture groups (very useful)
const re = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/
const match = '2018-06-02'.match(re)
const { year, month, day } = match.groups
// year='2018', month='06', day='02'

// s flag: . matches newlines
const html = '<div>\n  content\n</div>'
/<div>(.*)<\/div>/s.test(html)  // true (previously required [\s\S])

// Lookbehind assertion
/(?<=\$)\d+/.exec('$100')  // matches 100 (previously only lookahead was available)

Summary

• Promise.finally simplifies cleanup logic
• Object spread/rest is now an official standard
• for await...of is very convenient for handling async sequences
• Named capture groups greatly improve regex readability