JavaScript Numeric Separators (`_`)

Array.prototype.at() returns the element at a given integer index. Its main advantage over bracket notation is support for negative indexes. Positive Indexes Copy const fruits = ['apple', 'banana', 'cherry']; console.log(fruits.at(0)); // 'apple' console.log(fruits.at(1)); // 'banana' Negative Indexes Copy console.log(fruits.at(-1)); // 'cherry' console.log(fruits.at(-2)); // 'banana' Without at(), the traditional equivalent for the last element is:

String.prototype.matchAll() returns an iterator containing every match of a global regular expression, including capturing groups and match indexes. Copy const text = 'Order #123, Order #456, Order #789'; const regex = /Order #(\d+)/g; for (const match of text.matchAll(regex)) { console.log(match[0], match[1], match.index); } Named Capturing Groups Copy const text = 'John: 123, Jane: 456'; const regex = /(?<name>\w+): (?<number>\d+)/g; for (const match of text.matchAll(regex)) { console.log(match.groups.name, match.groups.number); } Convert the Iterator to an Array Copy const matches = [...'cat bat mat'.matchAll(/\b(\w+)at\b/g)]; This is convenient when you need map, filter, or other array operations, but iterating directly avoids storing all matches at once.

Promise.allSettled() waits until every input promise has either fulfilled or rejected, then returns the outcome of each operation. Unlike Promise.all(), one rejection does not stop you from receiving the other results. Copy const tasks = [ Promise.resolve('first result'), Promise.reject(new Error('second failed')), Promise.resolve('third result'), ]; const results = await Promise.allSettled(tasks); for (const result of results) { if (result.status === 'fulfilled') { console.log('Value:', result.value); } else { console.error('Error:', result.reason); } } Each result has one of these shapes:

JavaScript Number can represent integers exactly only up to Number.MAX_SAFE_INTEGER (2^53 - 1). When you need larger integer values without losing precision, use BigInt. Create a BigInt Copy const a = 123456789012345678901234567890n; const b = BigInt('123456789012345678901234567890'); The n suffix creates a BigInt literal. BigInt() is useful when converting a string or an integer-valued Number.

The nullish coalescing operator, ??, provides a fallback only when the left-hand value is null or undefined. Copy const username = null; const name = username ?? 'Guest'; console.log(name); // Guest ?? vs. || || falls back for every falsy value. ?? preserves valid falsy data such as 0, false, and '':

The JavaScript regular-expression d flag enables match indices. It does not collect repeated captures; instead, it adds an indices property that tells you the start and end offsets of the overall match and each capturing group. Basic Example Copy const regex = /(?<year>\d{4})-(?<month>\d{2})-(?<day>\d{2})/d; const match = regex.exec('Date: 2024-08-30'); console.log(match[0]); // 2024-08-30 console.log(match.indices[0]); // [6, 16] console.log(match.indices.groups.year); // [6, 10] console.log(match.indices.groups.month); // [11, 13] console.log(match.indices.groups.day); // [14, 16] The end index is exclusive, matching the behavior of String.prototype.slice().