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Web-shaped APIs

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zig-js is not a browser and ships no DOM. It does implement the host-shaped APIs that server-side runtimes and embedders expect, so a JavaScript runtime built on this engine does not have to reimplement them.

Timers

Context-owned setTimeout, clearTimeout, setInterval, clearInterval. Scheduling returns immediately; callbacks run only on the event loop that created the timer. Handles are stable objects with ref(), unref(), hasRef(), and refresh(). Full details: Timers.

URLs

URL and URLSearchParams, including origin/host/port parsing, percent encoding, and IDNA handling via the checked-in unicode_idna_data.zig tables.

Text encoding

TextEncoder and TextDecoder over the generated codec tables (text_codec.zig, text_codec_tables.zig, encoding_multibyte*.zig, encoding_singlebyte_data.zig), covering the single-byte and multi-byte legacy encodings alongside UTF-8, with the standard streaming and fatal/replacement behaviours.

btoa and atob provide base64 at the global level.

Fetch data types

Headers, Request, Response, FormData, and Blob are implemented as data types — header normalization and guard rules, body consumption (text/json/arrayBuffer/blob/formData), multipart parsing, and Blob slicing. Header semantics live in src/fetch_headers.zig.

There is no network stack. These types exist so an embedder can wire its own transport to standard shapes; the engine performs no I/O.

Cancellation

AbortController and AbortSignal, including AbortSignal.timeout() and the abort-reason propagation that consumers check. Signal timeout generations are tracked by the Context, so a torn-down context cannot fire a stale abort.

Structured clone

structuredClone() uses the same wire format as Worker postMessage (src/structured_clone.zig): cyclic graphs, Map/Set/Date/RegExp/Error, typed arrays and DataView, ArrayBuffer transfer (the source detaches), and SharedArrayBuffer retention (shared, not copied).

Microtasks and the run loop

queueMicrotask shares the promise-reaction queue. The host drains microtasks at defined checkpoints; timers are a separate macrotask stage. Harness globals drainMicrotasks, $drainRunLoop, and $drainFinalizationCleanup let a test host step the loop deterministically — they are harness surface, not stable embedder API.

What an embedder still owns

  • Networking, filesystem, process, and stdio.
  • Module specifier resolution and loading (Context.evaluateModule takes a host hook — see Embedding).
  • Logging: there is no console.
  • The event-loop policy itself. The engine exposes the queues and the drain points; the run loop is the host's.