What’s New in JavaScript (ES2026) Cheat Sheet
This page condenses the finished JavaScript features in ES2026, plus a recap of ES2025, into one scannable reference. You can also download the information as a printable cheat sheet:
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Get the What's New in JavaScript (ES2026) Cheat Sheet (PDF), a printable reference to Map upsert, JSON.parse source text, Iterator.concat, Array.fromAsync, Base64 and hex bytes, Error.isError, and an ES2025 recap:
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Upgrading
- ES2026 covers proposals finished (Stage 4) for the 2026 spec
- Check engines before you ship: Node, browsers
- Temporal and
usingare Stage 4 but land in ES2027
Run the Latest Node.js
$ node --version
v26.13.0
$ npx node@26 script.mjs
Test for a Feature
typeof Math.sumPrecise; // "undefined"
typeof Iterator.concat; // "function"
Map Upsert
getOrInsertreplaces thehas,set,getdancegetOrInsertComputedruns the callback only if the key is missingWeakMaphas both methods too
Insert a Default Once
const groups = new Map();
groups.getOrInsert("a", []).push(1);
groups.getOrInsert("a", []).push(2);
groups; // Map(1) { 'a' => [ 1, 2 ] }
Compute a Default Lazily
const m = new Map();
m.getOrInsertComputed("b", (k) => k + "!");
m; // Map(1) { 'b' => 'b!' }
Iterator.concat
- Chains iterables lazily into one iterator
Join Iterables
Iterator.concat([1, 2], new Set([3]))
.map((n) => n * 2)
.toArray(); // [ 2, 4, 6 ]
JSON.parse Source Text
Keep Big Integers Exact
JSON.parse(
'{"id":12345678901234567890}',
(key, val, { source }) =>
typeof val === "number"
? BigInt(source)
: val,
);
// { id: 12345678901234567890n }
Write a Number Verbatim
JSON.stringify({
n: JSON.rawJSON("12345678901234567890"),
});
// '{"n":12345678901234567890}'
Free Bonus: Download the What’s New in JavaScript (ES2026) Cheat Sheet PDF and keep the essentials at hand.
Array.fromAsync
Collect an Async Iterable
async function* gen() {
yield 1;
yield 2;
}
await Array.fromAsync(gen());
// [ 1, 2 ]
Await Promises and Map Them
await Array.fromAsync(
[Promise.resolve(1), 2],
(n) => n * 2,
); // [ 2, 4 ]
Base64 and Hex Bytes
- No more
btoaandBufferworkarounds - Works on any
Uint8Array, in browsers too
Encode and Decode
const bytes = new Uint8Array([72, 105]);
bytes.toBase64(); // "SGk="
bytes.toHex(); // "4869"
Uint8Array.fromBase64("SGk=");
// Uint8Array(2) [ 72, 105 ]
Uint8Array.fromHex("ff00");
// Uint8Array(2) [ 255, 0 ]
URL-Safe Base64
new Uint8Array([251, 255]).toBase64({
alphabet: "base64url",
}); // "-_8="
Error.isError
Check Errors Reliably
Error.isError(new TypeError("x")); // true
Error.isError({ message: "x" }); // false
Math.sumPrecise
Not in Node 26 Yet
// Stage 4 for ES2026, but Node 26 lacks it
typeof Math.sumPrecise; // "undefined"
// Once shipped: exact, no drift
// Math.sumPrecise([0.1, 0.2, 0.3])
Free Bonus: Download the What’s New in JavaScript (ES2026) Cheat Sheet PDF and keep the essentials at hand.
ES2025 Recap: Iterators and Sets
Iterator Helpers
[1, 2, 3, 4].values()
.filter((n) => n % 2)
.map((n) => n * 10)
.take(2)
.toArray(); // [ 10, 30 ]
Set Methods
const a = new Set([1, 2, 3]);
const b = new Set([2, 3, 4]);
a.union(b); // Set(4) { 1, 2, 3, 4 }
a.intersection(b); // Set(2) { 2, 3 }
a.difference(b); // Set(1) { 1 }
a.isSubsetOf(b); // false
ES2025 Recap: Promises and Modules
Promise.try
Promise.try(() => {
throw new Error("x");
}).catch((e) => e.message);
// Promise resolves to "x"
JSON Modules
import data from "./d.json"
with { type: "json" };
data; // { a: 1 }
ES2025 Recap: Numbers
Float16 Typed Arrays
new Float16Array([1.337])[0];
// 1.3369140625
Math.f16round(1.337);
// 1.3369140625
ES2025 Recap: Regular Expressions
RegExp.escape
RegExp.escape("a.b*c"); // "\\x61\\.b\\*c"
Inline Modifiers
/(?i:a)b/.test("Ab"); // true
/(?i:a)b/.test("AB"); // false
Duplicate Named Groups
const re =
/(?<y>\d{4})-\d\d|\d\d-(?<y>\d{4})/;
"2026-10".match(re).groups.y; // "2026"
"10-2026".match(re).groups.y; // "2026"
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You can download this information as a printable cheat sheet:
Free Bonus: What's New in JavaScript (ES2026) Cheat Sheet
Get the What's New in JavaScript (ES2026) Cheat Sheet (PDF), a printable reference to Map upsert, JSON.parse source text, Iterator.concat, Array.fromAsync, Base64 and hex bytes, Error.isError, and an ES2025 recap:
