diff --git a/README.md b/README.md
index eb07a52..395fb89 100644
--- a/README.md
+++ b/README.md
@@ -7,13 +7,14 @@ import {formatNpmrc} from 'dotnpmrc'
 
 const contents = formatNpmrc({
   format: 'npm9',
-  scopes: [{
-    name: 'example',
-    registry: 'https://npm-registry.example.com/',
-    auth: 'base64-of-username-and-password',
-    authToken: 'npmToken.bearer',
-    alwaysAuth: true,
-  }],
+  scopes: {
+    example: {
+      registry: 'https://npm-registry.example.com/',
+      auth: 'base64-of-username-and-password',
+      authToken: 'npmToken.bearer',
+      alwaysAuth: true,
+    }
+  },
   root: {
     registry: 'https://registry.yarnpkg.com'
   }
diff --git a/package.json b/package.json
index d09d245..6594de2 100644
--- a/package.json
+++ b/package.json
@@ -40,7 +40,11 @@
     "type": "git",
     "url": "https://github.com/semrel-extra/npmrc.git"
   },
+  "dependencies": {
+    "ini": "^4.0.0"
+  },
   "devDependencies": {
+    "@types/ini": "^1.3.31",
     "@types/node": "^18.15.10",
     "c8": "^7.13.0",
     "concurrently": "^7.6.0",
diff --git a/src/main/ts/index.ts b/src/main/ts/index.ts
index 7212fb6..040ead5 100644
--- a/src/main/ts/index.ts
+++ b/src/main/ts/index.ts
@@ -1 +1,53 @@
-export const foo = 'bar'
\ No newline at end of file
+import {encode} from 'ini'
+
+export interface INpmrcData {
+  format?: string,
+  root?: Record<string, string>,
+  scopes?: Record<string, {
+    registry: string
+    auth?: string
+    authToken?: string
+    username?: string
+    password?: string
+    [i: string]: string | undefined
+  }>
+}
+
+const credKeys = new Set(['_auth', '_authToken', 'username', '_password'])
+const credAliases = new Set(['auth', 'authToken', 'password'])
+
+// https://docs.npmjs.com/cli/v9/configuring-npm/npmrc
+export const formatNpmrc = ({scopes = {}, root = {}, format}: INpmrcData): string => {
+  const config: Record<string, string> = {}
+  const registry = normalizeRegistry(root.registry || 'registry.npmjs.org')
+
+  for (const [name, scope] of Object.entries(scopes)) {
+    for (const [k, v] of Object.entries(scope)) {
+      if (k === 'registry') {
+        config[`@${normalizeScope(name)}:${k}`] = v as string
+        continue
+      }
+      config[`${normalizeRegistry(scope.registry)}:${normalizeKey(k)}`] =  v || 'true'
+    }
+  }
+
+  for (const [_k, v, k = normalizeKey(_k)] of Object.entries(root)) {
+    if (format === 'npm9' && credKeys.has(k)) {
+      config[`${registry}:${k}`] = v || 'true'
+      continue
+    }
+    config[k] = v || 'true'
+  }
+
+  return encode(config)
+}
+
+const normalizeRegistry = (value: string): string => value
+  .replace(/^(https?:\/\/)?/, '//')
+  .replace(/\/*$/, '/')
+
+const normalizeScope = (value: string): string => value.replace(/^@/, '')
+const normalizeKey = (value: string): string =>
+  credAliases.has(value)
+    ? `_${value}`
+    : value
diff --git a/src/test/js/index.cjs b/src/test/js/index.cjs
index ecfc3a6..1451d12 100644
--- a/src/test/js/index.cjs
+++ b/src/test/js/index.cjs
@@ -1,9 +1,9 @@
 const { test } = require('uvu')
 const assert = require('node:assert')
-const toposource = require('@semrel-extra/npmrc')
+const npmrc = require('@semrel-extra/npmrc')
 
 test('index (cjs)', () => {
-  assert.ok(typeof toposource.foo === 'string')
+  assert.ok(typeof npmrc.formatNpmrc === 'function')
 })
 
 test.run()
diff --git a/src/test/js/index.mjs b/src/test/js/index.mjs
index a81a013..3cb94da 100644
--- a/src/test/js/index.mjs
+++ b/src/test/js/index.mjs
@@ -1,9 +1,9 @@
 import { test } from 'uvu'
 import assert from 'node:assert'
-import {foo} from '@semrel-extra/npmrc'
+import { formatNpmrc } from '@semrel-extra/npmrc'
 
 test('index (mjs)', () => {
-  assert.ok(typeof foo === 'string')
+  assert.ok(typeof formatNpmrc === 'function')
 })
 
 test.run()
diff --git a/src/test/ts/index.test.ts b/src/test/ts/index.test.ts
index 583fa5b..19f71e8 100644
--- a/src/test/ts/index.test.ts
+++ b/src/test/ts/index.test.ts
@@ -1,10 +1,63 @@
 import { test } from 'uvu'
 import * as assert from 'uvu/assert'
 
-import { foo } from '../../main/ts'
+import {formatNpmrc, INpmrcData} from '../../main/ts'
 
 test('index has proper index', () => {
-  assert.ok(typeof foo === 'string')
+  assert.ok(typeof formatNpmrc === 'function')
+})
+
+test('`formatNpmrc()` returns proper contents', () => {
+  const cases: [INpmrcData, string][] = [
+    [
+      {},
+      ''
+    ],
+    [
+      {
+        scopes: {
+          test: {
+            registry: 'https://registry.example.com',
+            'always-auth': '',
+            auth: 'basic',
+            _authToken: 'bearer'
+          }
+        }
+      },
+`@test:registry=https://registry.example.com
+//registry.example.com/:always-auth=true
+//registry.example.com/:_auth=basic
+//registry.example.com/:_authToken=bearer
+`
+    ],
+    [
+      {
+        format: 'npm9',
+        root: {
+          'always-auth': '',
+          'auth': 'basic'
+        }
+      },
+`always-auth=true
+//registry.npmjs.org/:_auth=basic
+`
+    ],
+    [
+      {
+        root: {
+          'always-auth': '',
+          'auth': 'basic'
+        }
+      },
+`always-auth=true
+_auth=basic
+`
+    ]
+  ];
+
+  for (const [data, expected] of cases) {
+    assert.equal(formatNpmrc(data), expected)
+  }
 })
 
 test.run()
diff --git a/yarn.lock b/yarn.lock
index 967b6b8..92ebe44 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -243,6 +243,11 @@
     "@nodelib/fs.scandir" "2.1.5"
     fastq "^1.6.0"
 
+"@types/ini@^1.3.31":
+  version "1.3.31"
+  resolved "https://registry.yarnpkg.com/@types/ini/-/ini-1.3.31.tgz#c78541a187bd88d5c73e990711c9d85214800d1b"
+  integrity sha512-8ecxxaG4AlVEM1k9+BsziMw8UsX0qy3jYI1ad/71RrDZ+rdL6aZB0wLfAuflQiDhkD5o4yJ0uPK3OSUic3fG0w==
+
 "@types/istanbul-lib-coverage@^2.0.1":
   version "2.0.4"
   resolved "https://registry.yarnpkg.com/@types/istanbul-lib-coverage/-/istanbul-lib-coverage-2.0.4.tgz#8467d4b3c087805d63580480890791277ce35c44"
@@ -1395,6 +1400,11 @@ inherits@2:
   resolved "https://registry.yarnpkg.com/inherits/-/inherits-2.0.4.tgz#0fa2c64f932917c3433a0ded55363aae37416b7c"
   integrity sha512-k/vGaX4/Yla3WzyMCvTQOXYeIHvqOKtnqBduzTHpzpQZzAskKMhZ2K+EnBiSM9zGSoIFeMpXKxa4dYeZIQqewQ==
 
+ini@^4.0.0:
+  version "4.0.0"
+  resolved "https://registry.yarnpkg.com/ini/-/ini-4.0.0.tgz#35b4b0ba3bb9a3feb8c50dbf92fb1671efda88eb"
+  integrity sha512-t0ikzf5qkSFqRl1e6ejKBe+Tk2bsQd8ivEkcisyGXsku2t8NvXZ1Y3RRz5vxrDgOrTBOi13CvGsVoI5wVpd7xg==
+
 internal-slot@^1.0.3, internal-slot@^1.0.5:
   version "1.0.5"
   resolved "https://registry.yarnpkg.com/internal-slot/-/internal-slot-1.0.5.tgz#f2a2ee21f668f8627a4667f309dc0f4fb6674986"