diff --git a/README.md b/README.md
index b6da602..de92e60 100644
--- a/README.md
+++ b/README.md
@@ -1,2 +1,37 @@
 # npi-validator
-Validate NPI numbers for healthcare physicians or organizations
+
+Validation function for National Provider Identifier (NPI) numbers. Works for healthcare physicians and organizations in the United States or other countries that use the NPI standard.
+
+This is an implementation of the Luhn formula (mod 10 double add double) check digit, see the [CMS.gov specification](https://www.cms.gov/Regulations-and-Guidance/Administrative-Simplification/NationalProvIdentStand/Downloads/NPIcheckdigit.pdf) for more information.
+
+## Install
+
+```sh
+npm install npi-validator
+```
+
+## Usage
+
+```js
+import npiValid from 'npi-validator';
+
+const npi = 1234567893;
+const valid = npiValid(npi);
+
+console.log(valid);
+//=> true
+```
+
+## API
+
+### `function npiValid(id: string | number, prefix?: string | number): boolean`
+
+```js
+import npiValid from 'npi-validator';
+
+const npi = 1234567893;
+const valid = npiValid(npi);
+
+console.log(valid);
+//=> true
+```
diff --git a/index.d.ts b/index.d.ts
new file mode 100644
index 0000000..2d9f382
--- /dev/null
+++ b/index.d.ts
@@ -0,0 +1,18 @@
+/**
+Validates NPI numbers. See the [CMS.gov specification](https://www.cms.gov/Regulations-and-Guidance/Administrative-Simplification/NationalProvIdentStand/Downloads/NPIcheckdigit.pdf) for more information.
+
+@example
+```
+import npiValid from 'npi-validator';
+
+const npi = 1234567893;
+const valid = npiValid(npi);
+
+console.log(valid);
+//=> true
+```
+
+@param id - npi number; must be 10 digits
+@param prefix - prefix number; defaults to 80840; must start with 80,the health prefix, and be followed by the 3 digit country code
+*/
+export function npiValid(id: string | number, prefix?: string | number): boolean;
diff --git a/index.js b/index.js
new file mode 100644
index 0000000..49bd95b
--- /dev/null
+++ b/index.js
@@ -0,0 +1,47 @@
+const {ceil, floor} = Math;
+const toInt = s => Number.parseInt(s, 10);
+
+const sumDigits = int => {
+	let sum = 0;
+	while (int) {
+		sum += int % 10;
+		int = floor(int / 10);
+	}
+
+	return sum;
+};
+
+const doubleAlternate = (value, index) => {
+	let pos = index ?? value.length - 1;
+	let sum = 0;
+	let alt = true;
+	for (; pos >= 0; pos--) {
+		const cur = toInt(value.charAt(pos));
+		sum += alt ? sumDigits(cur * 2) : cur;
+		alt = !alt;
+	}
+
+	return sum;
+};
+
+export function npiValid(id, prefix = '80840') {
+	const prefixRegex = /^80\d{3}$/;
+	const npiRegex = /^\d{10}$/;
+	const usPrefixSum = 24; // Precalculated value for 80840 (United States)
+	const npi = id?.toString() ?? '';
+	const pre = prefix?.toString() ?? '';
+
+	if (!npiRegex.test(npi) || !prefixRegex.test(pre)) {
+		return false;
+	}
+
+	const check = toInt(npi.charAt(9));
+	const npiSum = doubleAlternate(npi, 8);
+	const preSum = pre === '80840' ? usPrefixSum : doubleAlternate(pre, 3);
+	const total = npiSum + preSum;
+	const totalCeil = ceil(total / 10) * 10;
+
+	return totalCeil - total === check;
+}
+
+export default npiValid;
diff --git a/package.json b/package.json
new file mode 100644
index 0000000..b2fbf7d
--- /dev/null
+++ b/package.json
@@ -0,0 +1,40 @@
+{
+	"name": "npi-validator",
+	"version": "1.0.0",
+	"description": "Validate NPI numbers for healthcare physicians or organizations.",
+	"homepage": "https://github.com/coryasilva/npi-validator#readme",
+	"bugs": {
+		"url": "https://github.com/coryasilva/npi-validator/issues"
+	},
+	"repository": {
+		"type": "git",
+		"url": "git+https://github.com/coryasilva/npi-validator.git"
+	},
+	"author": {
+		"name": "Cory Silva",
+		"url": "https://corysilva.com"
+	},
+	"keywords": [
+		"healthcare",
+		"npi",
+		"validation"
+	],
+	"engines": {
+		"node": ">=18"
+	},
+	"type": "module",
+	"sideEffects": false,
+	"main": "index.js",
+	"exports": {
+		"types": "./index.d.ts",
+		"default": "./index.js"
+	},
+	"scripts": {
+		"test": "xo && ava && tsc index.d.ts"
+	},
+	"devDependencies": {
+		"ava": "^6.0.1",
+		"typescript": "^5.3.3",
+		"xo": "^0.56.0"
+	}
+}
diff --git a/test.js b/test.js
new file mode 100644
index 0000000..2e17a68
--- /dev/null
+++ b/test.js
@@ -0,0 +1,38 @@
+/* eslint-disable unicorn/numeric-separators-style */
+import test from 'ava';
+import npiValid from './index.js';
+
+test('NPI validation with default United States prefix', t => {
+	t.is(npiValid(1234567893), true);
+	t.is(npiValid('1234567893'), true);
+	t.is(npiValid(1234567890), false);
+
+	// Bad inputs
+	t.is(npiValid(''), false);
+	t.is(npiValid(' '), false);
+	t.is(npiValid(undefined), false);
+	t.is(npiValid(null), false);
+	t.is(npiValid({}), false);
+	t.is(npiValid([]), false);
+	t.is(npiValid(true), false);
+	t.is(npiValid(false), false);
+	t.is(npiValid(), false);
+});
+
+test('NPI validation with custom prefix', t => {
+	t.is(npiValid(1234567894, 80123), true);
+	t.is(npiValid(1234567894, '80123'), true);
+	t.is(npiValid(1234567894, 8012), false);
+	t.is(npiValid(1234567894, 81840), false);
+
+	// Bad inputs
+	t.is(npiValid(1234567893, ''), false);
+	t.is(npiValid(1234567893, '     '), false);
+	t.is(npiValid(1234567893, '80   '), false);
+	t.is(npiValid(1234567893, undefined), true);
+	t.is(npiValid(1234567893, null), false);
+	t.is(npiValid(1234567893, {}), false);
+	t.is(npiValid(1234567893, []), false);
+	t.is(npiValid(1234567893, true), false);
+	t.is(npiValid(1234567893, false), false);
+});