fast-is-english-word

Blazingly fast English word lookup. ~42M ops/s, 555 KB on npm, zero runtime dependencies.

Works in Node.js, Bun, Deno, browsers, edge runtimes, and Cloudflare Workers — same import everywhere.

Uses an xor filter — a space-efficient probabilistic data structure that's smaller and faster than bloom filters. The entire 234k-word dictionary compresses into a 282 KB binary with only 3 array accesses per lookup.

Install

npm install fast-is-english-word

Usage

import { isWord } from "fast-is-english-word";

isWord("hello");     // true
isWord("world");     // true
isWord("asdfgh");    // false
isWord("Hello");     // true  (case-insensitive)
isWord("it's");      // false (alpha-only)

Works identically in Node.js, browser bundlers (webpack, vite, rollup), and edge runtimes. The right entry point is selected automatically via the package exports map.

Performance

Benchmark	ops/s	ns/op
Known word ("algorithm")	42,000,000	24
Non-word ("xjqkv")	76,000,000	13
Short word ("a")	140,000,000	7
Long word ("extraordinary")	30,000,000	34

npm run bench

How it works

An xor filter stores 8-bit fingerprints in a compact array. For each query, 3 positions are computed and the XOR of those 3 entries is compared against the word's fingerprint.

• Zero false negatives — if a word is in the dictionary, isWord always returns true
• False positive rate ≈ 0.39% (xor8) — ~1 in 256 non-words may return true
• O(1) lookup — exactly 3 array accesses, independent of dictionary size
• 282 KB — vs ~2.5 MB for a raw word list, or ~36 MB for a Set

Platform support

Environment	Entry	How it loads
Node.js (ESM)	dist/index.js	readFileSync of binary filter
Node.js (CJS)	dist/index.cjs	readFileSync of binary filter
Browser / bundler	dist/browser.js	Self-contained, filter embedded as base64
Deno / edge / workers	dist/browser.js	Self-contained, no filesystem needed

The correct entry is resolved automatically by the exports map — you always just write:

import { isWord } from "fast-is-english-word";

API

isWord(word: string): boolean

Returns true if word is a recognized English word.

• Case-insensitive (lowercased internally)
• Returns false for empty strings, numbers, hyphens, apostrophes, spaces
• Only ASCII a–z characters are supported

Configuration

The filter is pre-built with xor8 by default (smallest size). You can rebuild with different options:

Fingerprint size

Mode	FP rate	Filter size
xor8 (default)	≈ 0.39%	282 KB
xor16	≈ 0.0015%	563 KB

# Rebuild with 16-bit fingerprints (higher accuracy, 2x size)
npm run build:filter:xor16
npm run build

Custom word list

# Use any newline-separated word list
node scripts/build-filter.mjs --words=./my-words.txt

# Combine options
node scripts/build-filter.mjs --bits=16 --words=./my-words.txt

npm run build

Comparison

Package	Approach	Size	Lookup
fast-is-english-word	Xor filter	555 KB	24 ns
an-array-of-english-words	JSON array	3.4 MB	~ms (linear scan)
word-list	Text file	2.8 MB	user builds own
check-if-word	Giant regex	39.6 MB	slow

License

MIT