diff --git a/.eslintrc.json b/.eslintrc.json
deleted file mode 100644
index 71b1354..0000000
--- a/.eslintrc.json
+++ /dev/null
@@ -1,15 +0,0 @@
-{
-  "env": {
-    "node": true,
-    "es6": true,
-    "jest": true
-  },
-  "rules": {
-    "no-param-reassign": "off"
-  },
-  "parser": "babel-eslint",
-  "parserOptions": {
-    "sourceType": "module",
-    "allowImportExportEverywhere": true
-  }
-}
diff --git a/.github/workflows/node.yml b/.github/workflows/node.yml
index 1b9cb25..819c557 100644
--- a/.github/workflows/node.yml
+++ b/.github/workflows/node.yml
@@ -12,7 +12,7 @@ jobs:
     runs-on: ubuntu-latest
     strategy:
       matrix:
-        node-version: [18.x, 20.x]
+        node-version: [20.x, 22.x, 24.x]
     steps:
       - uses: actions/checkout@v5
       - name: Use Node.js ${{ matrix.node-version }}
diff --git a/CLAUDE.md b/CLAUDE.md
new file mode 100644
index 0000000..4cb73bc
--- /dev/null
+++ b/CLAUDE.md
@@ -0,0 +1,21 @@
+# CLAUDE.md
+
+This file provides guidance to Claude Code (claude.ai/code) when working with code in this repository.
+
+## What This Is
+
+`diff-lockfiles` is a CLI tool that diffs `package-lock.json` files across git commit ranges, showing which dependencies changed versions. Fork of [lock-diff](https://github.com/mxweaver/lock-diff). ES module project (`"type": "module"`).
+
+## Commands
+
+- **Test:** `NODE_OPTIONS=--experimental-vm-modules npm test`
+- **Run single test:** `NODE_OPTIONS=--experimental-vm-modules npx jest __tests__/index.test.js`
+- **Lint:** `npm run lint`
+- **Run CLI:** `./bin/diff-lockfiles.js <from> <to>` (e.g., `./bin/diff-lockfiles.js HEAD~1 HEAD`)
+
+## Architecture
+
+- `bin/diff-lockfiles.js` — CLI entry point using Commander. Runs `git diff` to find changed lockfiles between two refs, parses them via `git show`, then calls `diff()` and `print()`.
+- `lib/index.js` — Core logic. Exports `diff(oldLock, newLock, shallow)` which compares `.packages` entries using semver, `format(changes, options)` which returns formatted strings (table, json, markdown, or text), and `print(changes, options)` which formats and writes to stdout. Color support via chalk.
+- `__tests__/index.test.js` — Jest tests for `diff()` and `format()`.
+- `data/` — Fixture lockfiles (lodash version variants) for testing.
diff --git a/__tests__/index.test.js b/__tests__/index.test.js
index 5ac59da..fe50533 100644
--- a/__tests__/index.test.js
+++ b/__tests__/index.test.js
@@ -1,4 +1,4 @@
-import { diff } from '../lib/index.js';
+import { diff, format } from '../lib/index.js';
 
 describe('diff', () => {
   it('returns an empty object when given two empty objects', () => {
@@ -15,3 +15,53 @@ describe('diff', () => {
     expect(changes).toEqual({});
   });
 });
+
+describe('format', () => {
+  const changes = {
+    'node_modules/foo': ['1.0.0', '2.0.0'],
+    'node_modules/bar': [null, '1.0.0'],
+    'node_modules/baz': ['3.0.0', null],
+  };
+
+  it('returns empty string for empty changes with text format', () => {
+    expect(format({}, { format: 'text' })).toBe('');
+  });
+
+  it('returns empty object string for empty changes with json format', () => {
+    expect(format({}, { format: 'json' })).toBe('{}');
+  });
+
+  it('returns JSON string for json format', () => {
+    const result = format(changes, { format: 'json' });
+    expect(JSON.parse(result)).toEqual(changes);
+  });
+
+  it('returns text with arrows for text format', () => {
+    const result = format(changes, { format: 'text', color: false });
+    expect(result).toContain('node_modules/foo 1.0.0 -> 2.0.0');
+    expect(result).toContain('node_modules/bar added');
+    expect(result).toContain('node_modules/baz removed');
+  });
+
+  it('returns markdown table for markdown format', () => {
+    const result = format(changes, { format: 'markdown' });
+    expect(result).toContain('| Package');
+    expect(result).toContain('node_modules/foo');
+  });
+
+  it('returns table for table format', () => {
+    const result = format(changes, { format: 'table', title: '', color: false });
+    expect(result).toContain('package');
+    expect(result).toContain('node_modules/foo');
+  });
+
+  it('includes title in markdown format', () => {
+    const result = format(changes, { format: 'markdown', title: 'package-lock.json' });
+    expect(result).toContain('## package-lock.json');
+  });
+
+  it('defaults to text format', () => {
+    const result = format(changes, { format: 'unknown', color: false });
+    expect(result).toContain('node_modules/foo 1.0.0 -> 2.0.0');
+  });
+});
diff --git a/eslint.config.js b/eslint.config.js
new file mode 100644
index 0000000..9314f65
--- /dev/null
+++ b/eslint.config.js
@@ -0,0 +1,11 @@
+export default [
+  {
+    languageOptions: {
+      ecmaVersion: 'latest',
+      sourceType: 'module',
+    },
+    rules: {
+      'no-param-reassign': 'off',
+    },
+  },
+];
diff --git a/lib/index.js b/lib/index.js
index 61100af..1847fb5 100644
--- a/lib/index.js
+++ b/lib/index.js
@@ -39,45 +39,42 @@ export function diff(oldLock, newLock, shallow) {
   return changes;
 }
 
-function printJSON(changes) {
-  /* eslint-disable no-console */
-  console.log(JSON.stringify(changes));
+function formatJSON(changes) {
+  return JSON.stringify(changes);
 }
 
-function printText(changes, options) {
-  /* eslint-disable no-console */
+function formatText(changes, options) {
+  const lines = [];
 
   Object.entries(changes).forEach(([name, [oldVersion, newVersion]]) => {
     if (!oldVersion) {
       if (options.color) {
-        console.log(`${name} ${chalk.green('added')}`);
+        lines.push(`${name} ${chalk.green('added')}`);
       } else {
-        console.log(`${name} added`);
+        lines.push(`${name} added`);
       }
     } else if (!newVersion) {
       if (options.color) {
-        console.log(`${name} ${chalk.red('removed')}`);
+        lines.push(`${name} ${chalk.red('removed')}`);
       } else {
-        console.log(`${name} removed`);
+        lines.push(`${name} removed`);
       }
     } else if (!semver.eq(oldVersion, newVersion)) {
       if (options.color) {
         const color = semver.gt(oldVersion, newVersion)
           ? chalk.red
           : chalk.green;
-        console.log(`${name} ${color(`${oldVersion} -> ${newVersion}`)}`);
+        lines.push(`${name} ${color(`${oldVersion} -> ${newVersion}`)}`);
       } else {
-        console.log(`${name} ${oldVersion} -> ${newVersion}`);
+        lines.push(`${name} ${oldVersion} -> ${newVersion}`);
       }
     }
   });
 
-  /* eslint-enable no-console */
+  return lines.join('\n');
 }
 
-function printTable(changes, options) {
-  /* eslint-disable no-console */
-
+function formatTable(changes, options) {
   let data = Object.entries(changes)
     .map(([name, [oldVersion, newVersion]]) => ([
       name,
@@ -110,14 +107,10 @@ function printTable(changes, options) {
     data[0] = data[0].map((heading) => chalk.bold(heading));
   }
 
-  console.log(table(data));
-
-  /* eslint-disable no-console */
+  return table(data);
 }
 
-function printMarkdown(changes, options) {
-  /* eslint-disable no-console */
-
+function formatMarkdown(changes, options) {
   // Helper function to format version changes with markdown emphasis
   function formatVersionChange(oldVersion, newVersion) {
     if (!oldVersion) return `**${newVersion}** (added)`;
@@ -144,30 +137,35 @@ function printMarkdown(changes, options) {
     ])
   ];
 
+  const parts = [];
+
   // Add title if provided
   if (options.title && options.title !== '') {
-    console.log(`## ${options.title}\n`);
+    parts.push(`## ${options.title}\n`);
   }
 
-  console.log(markdownTable(tableData));
+  parts.push(markdownTable(tableData));
 
-  /* eslint-enable no-console */
+  return parts.join('\n');
 }
 
-export function print(changes, options) {
+export function format(changes, options) {
   switch (options.format) {
     case 'json':
-      printJSON(changes, options);
-      break;
+      return formatJSON(changes, options);
     case 'table':
-      printTable(changes, options);
-      break;
+      return formatTable(changes, options);
     case 'markdown':
-      printMarkdown(changes, options);
-      break;
+      return formatMarkdown(changes, options);
     case 'text':
     default:
-      printText(changes, options);
-      break;
+      return formatText(changes, options);
+  }
+}
+
+export function print(changes, options) {
+  const output = format(changes, options);
+  if (output) {
+    console.log(output);
   }
 }