yn

yn is a tiny, dependency-free npm utility for validating and normalizing yes/no-style answers. It recognizes a fixed set of case-insensitive truthy and falsy tokens (‘y’, ‘yes’, ‘t’, ‘true’, 1, ‘on’ vs. ‘n’, ‘no’, ‘f’, ‘false’, 0, ‘off’) and returns a strict boolean or undefined for anything else, so callers can distinguish ‘no answer’ from an explicit false.

Its standout feature is an opt-in lenient mode: a keyboard-distance scoring algorithm (in lenient.js) that tolerates common typos of ‘yes’ and ‘no’ — such as ‘ywa’ or ‘mo’ — by weighting nearby keys on a QWERTY layout. This makes it well suited for validating free-text answers to CLI prompts, where users often mistype short confirmations.

What You Get

• A single default export function, yn(input, options?), with a fully typed TypeScript signature
• Recognition of common truthy/falsy tokens across strings, numbers, and native booleans (‘y’/‘n’, ‘yes’/‘no’, ‘true’/‘false’, 1/0, ‘on’/‘off’)
• An opt-in lenient mode that uses keyboard-distance scoring to tolerate typos of ‘yes’ and ‘no’
• A default option to supply a fallback boolean when no match is found, with a runtime type check that throws if it isn’t a boolean
• Zero runtime dependencies and a tiny (35KB) footprint, published as a native ESM package

Common Use Cases

• Validating a user’s typed answer to a CLI confirmation prompt (e.g. ‘Continue? (y/n)’)
• Normalizing boolean-like environment variables or config file values (‘true’/‘1’/‘on’) into real booleans
• Coercing loosely-typed API or form input into a strict boolean with a safe fallback via the default option
• Gracefully handling typo’d yes/no answers in interactive prompts using the lenient option instead of re-prompting the user

Under The Hood

Architecture The package is deliberately minimal: index.js exports a single default function that normalizes the input to a string, tests it against two case-insensitive regular expressions for truthy and falsy tokens, and falls through to an optional lenient path in lenient.js before returning the default option. There is no class hierarchy, no internal state, and no configuration beyond the two options — the entire control flow is a short sequence of guard clauses, making the single exported function trivial to reason about and impossible to misuse structurally.

Tech Stack Written in plain modern JavaScript, published as native ESM ("type": "module") with a hand-written index.d.ts providing full TypeScript types via function overloads (one signature for when default is set, one for when it isn’t). It has zero runtime dependencies; ava and tsd are used for testing and type-testing respectively, and xo (an opinionated ESLint wrapper) enforces linting. No build step is needed since the source ships as-is.

Code Quality Tests in test.js use the ava runner and cover truthy cases, falsy cases, undefined/unrecognized cases, both lenient-mode edge cases, and the default option’s runtime type-check error path with dedicated assertions — giving the small surface area comprehensive coverage. index.test-d.ts adds compile-time type assertions via tsd. Naming is consistent and terse in the idiomatic style of this ecosystem’s Sindresorhus-maintained packages, error handling is explicit (a thrown TypeError for a malformed default option), and xo plus CI (GitHub Actions, inferred from .github/) enforce style automatically.

What Makes It Unique Most yes/no parsers stop at exact token matching; this package’s lenient mode implements a keyboard-distance scoring heuristic — per-letter proximity maps for the keys around ‘y’, ‘e’, ‘s’, ‘n’, and ‘o’ — to accept common single-key-slip typos (‘ywa’, ‘mo’) as valid answers without a full fuzzy-matching or edit-distance library. It’s a narrow, purpose-built heuristic rather than a general novel algorithm, but it is a genuinely uncommon feature for a package this small.