is-char

is-char is a focused utility for one job: checking whether a value is exactly one JavaScript UTF-16 code unit.

In many codebases, this check appears in validators, parsers, CLIs, text filters, and protocol handlers. Keeping it in a dedicated package makes that intent explicit and reusable across projects.

Why this package matters

• It standardizes a common validation rule used in input boundaries.
• It removes repeated ad-hoc checks from application code.
• It keeps behavior consistent across services and libraries.
• It is minimal, dependency-free, and safe to use in performance-sensitive paths.

Install

npm

npm i is-char

JSR

npx jsr add @arvid/is-char

Usage

import isChar from "is-char";

isChar("a"); // true
isChar("ab"); // false
isChar(""); // false
isChar(1); // false
isChar("a", { is: "a" }); // true
isChar("a", { is: "b" }); // false

API

isChar(value)

Returns true when:

• value is a string
• value.length === 1

Otherwise returns false.

isChar(value, { is })

If is is provided, isChar returns true only when:

• value is a single UTF-16 code unit string
• is is a single UTF-16 code unit string
• value === is

An omitted is value, or is: undefined, applies no matching constraint. Other invalid is values return false.

Character semantics

is-char deliberately uses JavaScript's simple string-length rule:

typeof value === "string" && value.length === 1

In this package, a char means exactly one UTF-16 code unit. This is not the same as a Unicode code point or a user-perceived grapheme cluster.

| Input | UTF-16 code units | Result | | --- | ---: | --- | | "a" | 1 | true | | "é" | 1 | true | | "e\\u0301" | 2 | false | | "😀" | 2 | false | | "ab" | 2 | false |

"e\\u0301" contains the letter e followed by a combining acute accent. It may be displayed as one user-perceived character, but it contains two UTF-16 code units, so this package returns false.

Astral characters and multi-code-unit sequences, including many emoji, return false. Single-code-unit symbols such as "♥" return true.

This package does not normalize strings, combine grapheme clusters, identify emoji, or coerce non-string values.

JSR and Deno

For npm, Node.js, and bundlers:

import isChar from "is-char";

For Deno through JSR:

import isChar from "jsr:@arvid/is-char";

Browser usage

The package is browser-compatible because the implementation is dependency-free and uses only standard JavaScript string operations. It does not access Node.js APIs, the filesystem, or the DOM.

With a bundler

Install the package and import it as an ESM module:

npm i is-char

import isChar from "is-char";

const input = document.querySelector("input").value;
const valid = isChar(input);

The package's exports entry points directly to its ESM implementation, so modern bundlers can include it in browser builds.

Direct browser import from a CDN

For a browser that supports JavaScript modules, import the published package through a CDN:

<script type="module">
  import isChar from "https://cdn.jsdelivr.net/npm/is-char/index.js";

  const value = document.querySelector("input").value;
  document.querySelector("#result").textContent = String(isChar(value));
</script>

For JSR users, an ESM CDN such as esm.sh can expose the JSR package to a browser:

<script type="module">
  import isChar from "https://esm.sh/jsr/@arvid/is-char";

  console.log(isChar("a"));
</script>

Browser support depends on the browser supporting JavaScript modules. The package itself does not require a framework, bundler, polyfill, or runtime adapter.

TypeScript

import isChar from "is-char";

const value: unknown = "x";

if (isChar(value)) {
  console.log(`Single UTF-16 code unit: ${value}`);
}

Type definitions are included out of the box.