dtrexp-wasm

This module is ESM 🔆. Please read this.

DTRExp (read: "DTR Expression") evaluated by the Rust core, compiled to WebAssembly for JS hosts — browsers, Node, Deno, edge runtimes.

T0900:1800 E1:5          Mon–Fri, 09:00–18:00
E7#-1 M4                 last Sunday of April, every year
20200106/10D             every 10 days from 2020-01-06 (cron can't say this)
D13 E5                   every Friday the 13th (cron can't say this either)

Note that 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

The wasm binary (~63 KB, ~29 KB gzipped) ships inside the package and instantiates at import; no fetch configuration, no separate asset step.

Usage

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

const dtr = parse('T0900:1800 E1:5'); // business hours
dtr.covers(new Date(), { tz: 'Europe/Berlin' }); // —> true
dtr.covers('2026-07-11T10:00:00Z'); // —> false (a Saturday, evaluated in UTC)

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

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

API

parse(expression)

• expression String — Required. The DTRExp source text.

returns 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)

• expression String — Required. The DTRExp source text.

returns { valid, errors, warnings }; every issue carries a message and a 0-based position. An expression can be valid and warned; that is the point of the distinction.

DTRExp#covers(instant [, opts])

• instant Date | number | string | { epochMilliseconds } — Required. The instant to test.
• opts.tz String — Optional. An IANA time-zone identifier (e.g. "Europe/Berlin"). Default: "UTC".

returns Boolean.

An unknown zone throws a RangeError; exactly what Intl itself throws for one.

DTRExp#warnings

The spec §9.1 unsatisfiability warnings of a parsed expression; same content as validate(source).warnings, exposed on the instance so code that parses directly doesn't lose them.

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.

Related Projects

• dtrexp (spec) — the DTRExp specification (grammar, semantics, conformance vectors) this package implements.
• dtrexp-rs — the Rust core this package compiles to WebAssembly.
• dtrexp-js — the reference implementation; the pure-TypeScript alternative on npm.
• dtrexp-py · dtrexp-go · dtrexp-swift · dtrexp-java — the other implementations; same core interface.

License

MIT — © 2026, Onur Yıldırım.