JavaScript support

Scripts are modern JavaScript - the current ECMAScript standard library, minus a small set of exclusions that exist to keep evaluation deterministic and sandboxed. If a construct is ordinary JavaScript and not listed below, it works: classes, arrow functions, destructuring, template literals, iterators, Map/Set, typed arrays, JSON, the full Math object and friends.

The module shape#

A script is a module. Its top level runs ONCE when the script compiles - the place for constants, helper functions and module-level handles - and the exported frame() runs once per evaluated frame:

export const outputs = ["layer_1:transform.position"];

const RADIUS = 120;

export function frame({ time, dt, frame, fps }) {
  return [320 + Math.cos(time) * RADIUS, 180 + Math.sin(time) * RADIUS];
}

The argument to frame() carries the clock: time (seconds), dt (one frame in seconds), frame (the integer frame) and fps. The same values are available as globals, with ONE caution: inside frame() the bare word frame names the function itself, so destructure { frame } from the argument whenever you need the frame number in the body.

Excluded on purpose#

• eval and new Function - no code from strings; both throw immediately.
• WeakRef and FinalizationRegistry - garbage-collector timing must never influence a frame.
• SharedArrayBuffer and Atomics - no shared memory, no timing channels.
• Date.now() and the real clock - Date reports the comp's time; the time zone is pinned to UTC on every host.
• Unseeded Math.random - the global is replaced by the seeded stream, the same numbers on every machine (see Determinism).
• Host surfaces - no fetch, no DOM, no workers, no storage. typeof window is "undefined", exactly as in any plain sandbox.

Absent libraries#

Intl and Temporal are not present - locale tables and calendar databases would make renders differ between machines. Format numbers with the standard tools instead (toFixed, padStart, template literals - the Formatting page collects the recipes), and do date maths in frames and seconds.

Promises settle inside the frame#

async helpers and promises work, but everything must SETTLE within the frame being evaluated - a frame's value cannot arrive later. A frame() whose returned promise never settles reports the typed async_unsettled error. In practice: use plain synchronous code in frame(); there is nothing asynchronous worth awaiting inside a sandbox with no outside world.

Budgets, not trust#

The runtime does not rely on scripts being polite. Poll budgets stop runaway loops, a memory ceiling stops runaway allocation and a stack guard stops runaway recursion - each reported as a typed status while the frame renders with base values. The numbers live on the Budgets page.