WASM

dtrexp-wasm is DTRExp evaluated by the Rust core, compiled to WebAssembly for JS hosts — browsers, Node, Deno, edge runtimes. The wasm binary (~63 KB, ~29 KB gzipped) ships inside the package and instantiates at import; no fetch configuration, no separate asset step.

For most JS applications the reference implementation, dtrexp, is the right pick; it is pure TypeScript, zero-dependency, and implements the full Tier-2 API (next(), describe(), toRRule()…). This package exists for a different job: running the Rust evaluator itself in a JS host. Use it when your backend already standardizes on dtrexp-rs and you want the same binary logic in the browser, or for cross-implementation parity checks against the reference.

Install

npm i dtrexp-wasm

Quick Start

import { parse, validate } from 'dtrexp-wasm';

const dtr = parse('T0900:1800 E1:5'); // business hours

// coverage — evaluated by the wasm core, one call per check
dtr.covers(new Date(), { tz: 'Europe/Berlin' });
// —> true (weekday, 09:00–18:00 Berlin local time)

dtr.covers('2026-07-11T10:00:00Z');
// —> false (a Saturday, evaluated in UTC)

// non-throwing validation, with the spec's unsatisfiability lint
validate('D30 M2');
// —> { valid: true, errors: [], warnings: [{ message: '…unsatisfiable…', position: 0 }] }

The operation vocabulary is fixed across every DTRExp implementation (API reference); if you know one library, you know this one.

Examples

An access-control grant that only applies during business hours; the expression lives in the grant as data, and the check runs on every request:

import { parse } from 'dtrexp-wasm';

// grant.scope = "T0900:1800 E1:5"  — stored in your DB / ACL
const scope = parse(grant.scope);

function authorize(req) {
  if (!scope.covers(Date.now(), { tz: req.user.tz })) {
    throw new ForbiddenError('outside the permitted window');
  }
  // … proceed
}

Validation with the spec’s unsatisfiability lint; D30 M2 parses but can never match, and the library tells you so instead of failing silently:

import { validate } from 'dtrexp-wasm';

validate('D30 M2');
// —> { valid: true, errors: [], warnings: [{ message: '…unsatisfiable…', position: 0 }] }

An expression can be valid and warned; that is the point of the distinction. When you parse directly, the same warnings are on the instance:

parse('D30 M2').warnings;
// —> [{ message: '…unsatisfiable…', position: 0 }]

API

Functions

Function	Description
`parse(expression)`	Parses a DTRExp string into a `DTRExp` instance. Fails with a positioned `DTRExpSyntaxError` (`position`, `expression` properties) on invalid input. Parse failure is the only failure; a syntactically valid expression never fails later.
`validate(expression)`	Non-throwing variant. Returns `{ valid, errors, warnings }`; every issue carries a `message` and a 0-based `position`. Warnings include the spec’s unsatisfiability lint.

`DTRExp` Instance

Member	Description
`covers(instant, opts?)`	Whether the expression covers the instant. Returns `Boolean`. An unknown zone throws a `RangeError`; exactly what `Intl` itself throws for one.
`warnings`	The §9.1 unsatisfiability warnings of the parsed expression; same content as `validate(source).warnings`, exposed on the instance so code that parses directly doesn’t lose them.

Inputs & Options

Instants: Date, epoch milliseconds, ISO 8601 string, or any Temporal-like object exposing epochMilliseconds (no Temporal dependency).
opts.tz: IANA time zone for evaluation (e.g. "Europe/Berlin"), default "UTC".

Time Zones

WebAssembly has no zoneinfo filesystem, so this package does not ship a time-zone database. Zone lookups are answered by the host’s own IANA data through Intl.DateTimeFormat — the same technique the reference implementation uses — which keeps the binary small and the zone data exactly as current as the runtime’s. UTC (the default) never touches the bridge and evaluates entirely inside the wasm module.

Conformance

The spec’s test vectors are the contract (spec §12); this package vendors vectors.json and runs all of it in its test suite — 97 groups, 409 checks, including the coverage groups evaluated through the Intl zone bridge.