Add type 'guess'

README.md

@@ -42,10 +42,10 @@ fromMem(code: string, options: FromMemOptions): Promise<unknown>
 
 export type FromMemOptions = {
     /**
-     * What format does the code have?  Throws an error if the format is not
-     * "commonjs", "es", "umd", or "bare".  Default: "commonjs".
+     * What format does the code have?  "guess" means to read the closest
+     * package.json file looking for the "type" key.  Default: "commonjs".
      */
-    format?: "amd" | "bare" | "commonjs" | "es" | "globals" | "umd";
+    format?: "bare" | "commonjs" | "es" | "globals" | "guess";
     /**
      * What is the fully-qualified synthetic
      * filename for the code?  Most important is the directory, which is used to

index.js

@@ -6,10 +6,11 @@
 // Ideas taken from the "module-from-string" and "eval" modules, neither of
 // which were situated correctly to be used as-is.
 
-const vm = require("vm");
-const { Module } = require("module");
-const path = require("path");
-const url = require("url");
+const fs = require("node:fs/promises");
+const vm = require("node:vm");
+const { Module } = require("node:module");
+const path = require("node:path");
+const url = require("node:url");
 const semver = require("semver");
 
 // These already exist in a new, blank VM.  Date, JSON, NaN, etc.
@@ -40,9 +41,9 @@ globalContext.console = console;
  * Options for how to process code.
  *
  * @typedef {object} FromMemOptions
- * @property {"amd"|"bare"|"commonjs"|"es"|"globals"|"umd"} [format="commonjs"]
- *   What format does the code have?  Throws an error if the format is not
- *   "commonjs", "es", "umd", or "bare".
+ * @property {"amd"|"bare"|"commonjs"|"es"|"globals"|"guess"|"umd"} [format="commonjs"]
+ *   What format does the code have?  "guess" means to read the closest
+ *   package.json file looking for the "type" key.
  * @property {string} filename What is the fully-qualified synthetic
  *   filename for the code?  Most important is the directory, which is used to
  *   find modules that the code import's or require's.
@@ -117,7 +118,7 @@ async function importString(code, dirname, options) {
     ? options.filename
     : url.pathToFileURL(options.filename).toString();
   const dirUrl = dirname.startsWith("file:")
-    ? dirname
+    ? dirname + "/"
     : url.pathToFileURL(dirname).toString() + "/";
 
   const mod = new vm.SourceTextModule(code, {
@@ -150,6 +151,41 @@ async function importString(code, dirname, options) {
   return mod.namespace;
 }
 
+/**
+ * Figure out the module type for the given file.  If no package.json is
+ * found, default to "commonjs".
+ *
+ * @param {string} filename Fully-qualified filename to start from.
+ * @returns {Promise<"commonjs"|"es">}
+ * @throws On invalid package.json
+ */
+async function guessModuleType(filename) {
+  const fp = path.parse(filename);
+  switch (fp.ext) {
+    case ".cjs": return "commonjs";
+    case ".mjs": return "es";
+    default:
+      // Fall-through
+  }
+  let dir = fp.dir;
+  let prev = undefined;
+  while (dir !== prev) {
+    try {
+      const pkg = await fs.readFile(path.join(dir, "package.json"), "utf8");
+      const pkgj = JSON.parse(pkg);
+      return (pkgj.type === "module") ? "es" : "commonjs";
+    } catch (err) {
+      // If the file just didn't exist, keep going.
+      if (/** @type {NodeJS.ErrnoException} */ (err).code !== "ENOENT") {
+        throw err;
+      }
+    }
+    prev = dir;
+    dir = path.dirname(dir);
+  }
+  return "commonjs";
+}
+
 /**
  * Import or require the given code from memory.  Knows about the different
  * Peggy output formats.  Returns the exports of the module.
@@ -158,7 +194,6 @@ async function importString(code, dirname, options) {
  * @param {FromMemOptions} options Options.  Most important is filename.
  * @returns {Promise<unknown>} The evaluated code.
  */
-// eslint-disable-next-line require-await -- Always want to return a Promise
 module.exports = async function fromMem(code, options) {
   options = {
     format: "commonjs",
@@ -180,12 +215,18 @@ module.exports = async function fromMem(code, options) {
   // @ts-expect-error Context is always non-null
   options.context.globalThis = options.context;
 
-  if (!options?.filename) {
+  if (!options.filename) {
     throw new TypeError("filename is required");
   }
-  options.filename = path.resolve(options.filename);
+  if (!options.filename.startsWith("file:")) {
+    // File URLs must be already resolved.
+    options.filename = path.resolve(options.filename);
+  }
   const dirname = path.dirname(options.filename);
 
+  if (options.format === "guess") {
+    options.format = await guessModuleType(options.filename);
+  }
   switch (options.format) {
     case "bare":
     case "commonjs":

test/fixtures/bad/package.json

@@ -0,0 +1 @@
+{

test/fixtures/example.cjs

@@ -0,0 +1,6 @@
+"use strict";
+
+exports.foo = function foo() {
+  return 7;
+};
+

test/fixtures/mjs/package.json

@@ -0,0 +1,3 @@
+{
+  "type": "module"
+}

test/index.test.js

@@ -2,7 +2,8 @@
 
 const assert = require("node:assert");
 const fromMem = require("../index.js");
-const { join } = require("node:path");
+const { join, parse } = require("node:path");
+const { pathToFileURL } = require("node:url");
 const test = require("node:test");
 
 test("options", async() => {
@@ -19,15 +20,62 @@ test("commonjs", async() => {
     filename: "test1.js",
   });
   assert.equal(cjs, 4);
-  await assert.rejects(() => fromMem("throw new Error('foo')", {
+
+  const cjs2 = await fromMem(`
+const {foo} = require('./fixtures/example.cjs');
+module.exports = foo() + 2`, {
     filename: join(__dirname, "test2.js"),
+  });
+  assert.equal(cjs2, 9);
+
+  await assert.rejects(() => fromMem("throw new Error('foo')", {
+    filename: join(__dirname, "test3.js"),
     format: "bare",
   }), (/** @type {Error} */ err) => {
-    assert(/test2\.js/.test(err.stack), err.stack);
+    assert(/test3\.js/.test(err.stack), err.stack);
     return true;
   });
 });
 
+test("guess", async() => {
+  const cjs = await fromMem("module.exports = 4", {
+    filename: join(__dirname, "test_guess1.cjs"),
+    format: "guess",
+  });
+  assert.equal(cjs, 4);
+
+  const cjs2 = await fromMem("module.exports = 4", {
+    filename: join(__dirname, "test_guess2.js"),
+    format: "guess",
+  });
+  assert.equal(cjs2, 4);
+
+  // Hope there is not package.json in your root directory.  If there is,
+  // it better be commonjs. :)
+  const cjs3 = await fromMem("module.exports = 4", {
+    filename: join(parse(__dirname).root, "test_guess2.js"),
+    format: "guess",
+  });
+  assert.equal(cjs3, 4);
+
+  const mjs = await fromMem("export default 4", {
+    filename: join(__dirname, "test_guess3.mjs"),
+    format: "guess",
+  });
+  assert.equal(mjs.default, 4);
+
+  const mjs2 = await fromMem("export default 4", {
+    filename: join(__dirname, "fixtures", "mjs", "test_guess4.js"),
+    format: "guess",
+  });
+  assert.equal(mjs2.default, 4);
+
+  await assert.rejects(() => fromMem("export default 4", {
+    filename: join(__dirname, "fixtures", "bad", "test_guess5.js"),
+    format: "guess",
+  }));
+});
+
 test("esm", async() => {
   const mjs4 = await fromMem("export default 5", {
     filename: join(__dirname, "test4.js"),
@@ -57,4 +105,11 @@ export default foo();`, {
     format: "es",
   });
   assert.equal(mjs7.default, 6);
+
+  const mjs8 = await fromMem(`
+export default 8`, {
+    filename: pathToFileURL(join(__dirname, "test8.js")).toString(),
+    format: "es",
+  });
+  assert.equal(mjs8.default, 8);
 });