add an ESM version

PR-URL: #266
Credit: @eugene1g
Close: #266
Reviewed-by: @isaacs

EDIT(@isaacs): edited to move the test into a test file, and run the
build before running tests, and specify TS types explicitly for both
export types, and document the hybridness.

.gitignore

@@ -3,3 +3,4 @@
 /nyc_output
 /coverage
 /bundle
+/index.mjs

README.md

@@ -30,7 +30,10 @@ npm install lru-cache --save
 ## Usage
 
 ```js
-const LRU = require('lru-cache')
+// hybrid module, either works
+import LRUCache from 'lru-cache'
+// or:
+const LRUCache = require('lru-cache')
 
 // At least one of 'max', 'ttl', or 'maxSize' is required, to prevent
 // unsafe unbounded storage.
@@ -70,7 +73,7 @@ const options = {
   fetchMethod: async (key, staleValue, { options, signal }) => {}
 }
 
-const cache = new LRU(options)
+const cache = new LRUCache(options)
 
 cache.set("key", "value")
 cache.get("key") // "value"

package.json

@@ -10,7 +10,8 @@
   ],
   "sideEffects": false,
   "scripts": {
-    "build": "",
+    "pretest": "node ./scripts/transpile-to-esm.mjs",
+    "presnap": "node ./scripts/transpile-to-esm.mjs",
     "size": "size-limit",
     "test": "tap",
     "snap": "tap",
@@ -20,7 +21,23 @@
     "format": "prettier --write .",
     "typedoc": "typedoc ./index.d.ts"
   },
-  "main": "index.js",
+  "type": "commonjs",
+  "main": "./index.js",
+  "module": "./index.mjs",
+  "types": "./index.d.ts",
+  "exports": {
+    ".": {
+      "import": {
+        "types": "./index.d.ts",
+        "default": "./index.mjs"
+      },
+      "require": {
+        "types": "./index.d.ts",
+        "default": "./index.js"
+      }
+    },
+    "./package.json": "./package.json"
+  },
   "repository": "git://github.com/isaacs/node-lru-cache.git",
   "devDependencies": {
     "@size-limit/preset-small-lib": "^7.0.8",
@@ -41,6 +58,7 @@
   "license": "ISC",
   "files": [
     "index.js",
+    "index.mjs",
     "index.d.ts"
   ],
   "engines": {

scripts/transpile-to-esm.mjs

@@ -0,0 +1,9 @@
+#!/usr/bin/env node
+
+import { readFile, writeFile } from 'node:fs/promises'
+const cjs = await readFile(
+  new URL('../index.js', import.meta.url),
+  'utf8'
+)
+const esm = cjs.replace(/module.exports\s*=\s*/, 'export default ')
+await writeFile(new URL('../index.mjs', import.meta.url), esm, 'utf8')

test/esm-load.mjs

@@ -0,0 +1,10 @@
+import t from 'tap'
+import LRUCache from '../index.mjs'
+const c = new LRUCache({ max: 2 })
+t.type(c, LRUCache)
+c.set(1, 1)
+c.set(2, 2)
+c.set(3, 3)
+t.equal(c.get(1), undefined)
+t.equal(c.get(2), 2)
+t.equal(c.get(3), 3)