Built-ins

Every built-in constructor and prototype method is written from scratch in Zig (src/builtins.zig and the built-in installers in src/interpreter.zig). Nothing is delegated to a host JavaScript engine.

Global object

globalThis is the global object; global var and function declarations become own properties of it, while global lexical declarations do not.

Value properties: NaN, Infinity, undefined.

Function properties: eval, parseInt, parseFloat, isNaN, isFinite, encodeURI, encodeURIComponent, decodeURI, decodeURIComponent, escape, unescape, queueMicrotask, structuredClone, btoa, atob.

Harness and host hooks are also installed: print, gc, quit, drainMicrotasks, $drainRunLoop, $drainFinalizationCleanup, noInline, and numberOfDFGCompiles. These exist so conformance and engine-shell corpora run unmodified; treat them as harness surface, not as a stable embedding API. There is no console — embedders install their own.

Fundamental objects

Strings are UTF-16

String semantics are UTF-16: indexing, length, comparison, and iteration all operate on code units, with a separate code-point iterator for for…of. Internally strings are WTF-8 backed, so anything UTF-16-semantic must re-split astral scalars into surrogate pairs — a recurring source of subtle bugs, and the reason string work carries dedicated tests.

Unicode support is generated from checked-in tables: case mapping (unicode_case_data.zig), normalization (unicode_normalize_data.zig), grapheme segmentation (unicode_grapheme_data.zig), and IDNA (unicode_idna_data.zig, idna.zig). Regenerate them with the tools/gen_* scripts; never hand-edit.

Collections and keyed data

Map, Set, WeakMap, WeakSet, plus Map.groupBy and the ES2024 Set operations (union, intersection, difference, symmetricDifference, isSubsetOf, isSupersetOf, isDisjointFrom).

WeakRef and FinalizationRegistry are implemented against the precise collector: under enable_gc, collection clears WeakRef targets, prunes weak keys, queues registry records, and drains registered cleanup callbacks as host cleanup jobs at checkpoints. See Memory & GC.

Iteration

Symbol.iterator / Symbol.asyncIterator protocols throughout, plus the Iterator and AsyncIterator constructors with the iterator-helper method set described in Language.

Symbols

Symbol with the well-known symbols — iterator, asyncIterator, hasInstance, toPrimitive, toStringTag, species, unscopables, isConcatSpreadable, match/matchAll/replace/search/split, dispose, asyncDispose — plus the global symbol registry (Symbol.for / keyFor).

Well-known symbols are shared across realms, so symbol identity holds through $262.createRealm().

Promises and jobs

Promise with all, allSettled, any, race, resolve, reject, withResolvers, and try. The microtask queue lives in src/promise.zig and is owned by the Context.

Job ordering is exact and observable: queueMicrotask and promise reactions share one queue, and the host drains it at defined checkpoints. Timers are a separate macrotask stage — see Timers.

Reflection

Proxy with the full trap set, and Reflect with the matching operations. Proxy traps are honoured by the operations that commonly forget them here: object spread, object rest destructuring, Object.assign, Object.keys and friends, and public class field definition on a proxy receiver.

Binary data

ArrayBuffer, the twelve typed arrays, DataView, SharedArrayBuffer, and Atomics have their own page: Binary data.

Internationalization

Intl and Temporal have their own page: Intl & Temporal.

Host-shaped globals

setTimeout/setInterval, URL, TextEncoder/TextDecoder, Headers/Request/Response, Blob, FormData, AbortController, and structuredClone are documented in Web-shaped APIs.

WebAssembly is installed when the Wasm API is enabled for the context; see WebAssembly.