Feat: Import functions that are written as ES Modules (#140)

* feat: add the ability to use a function written as an ES module

README.md

@@ -98,6 +98,28 @@ with the CLI.
 npx faas-js-runtime ./index.js
 ```
 
+### Functions as ES Modules
+
+Functions can be written and imported as ES modules with either the `.mjs` file extenstion or by adding the `type` property to the functions package.json and setting it to `module`.
+
+```js
+// index.mjs
+const handle = async function(context) => { ... };
+
+// Export the function
+export { handle };
+```
+
+If using the `type` property, the package.json might look something like this:
+```js
+{
+  "name": "moduleName",
+  "type": "module"
+}
+```
+
+
+
 ## Usage as a Module
 
 In the current working directory, there is an `index.js` file like this.

bin/cli.js

@@ -2,6 +2,7 @@
 const path = require('path');
 
 const { start, defaults } = require('../');
+const { loadFunction } = require('../lib/function-loader.js');
 const pkg = require('../package.json');
 
 const ON_DEATH = require('death')({ uncaughtException: true });
@@ -20,7 +21,7 @@ program.parse(process.argv);
 
 async function runServer(file) {
   const programOpts = program.opts();
-  
+
   try {
     let server;
     let options = {
@@ -29,7 +30,8 @@ async function runServer(file) {
     };
 
     const filePath = extractFullPath(file);
-    const code = require(filePath);
+    const code = await loadFunction(filePath);
+
     if (typeof code === 'function') {
       server = await start(code, options);
     } else if (typeof code.handle === 'function') {

lib/function-loader.js

@@ -0,0 +1,38 @@
+const path = require('path');
+
+// This is the way to import es modules inside a CJS module
+const dynamicImport = new Function('modulePath', 'return import(modulePath)');
+
+async function loadFunction(filePath) {
+  if (isESM(filePath)) {
+    code = await dynamicImport(filePath);
+  } else {
+    code = require(filePath);
+  }
+
+  return code;
+}
+
+// https://nodejs.org/dist/latest-v18.x/docs/api/packages.html#determining-module-system
+// An ESM module can be determined 2 ways
+// 1. has the mjs file extention
+// 2. type=module in the package.json
+function isESM(filePath) {
+  const pathParsed = path.parse(filePath);
+
+  if (pathParsed.ext === '.mjs') {
+    return true;
+  }
+
+  // find the functions package.json and see if it has a type field
+  const functionPkg = require(path.join(pathParsed.dir, 'package.json'));
+  if (functionPkg.type === 'module') {
+    return true;
+  }
+
+  // Should default to CJS
+  return false;
+}
+
+
+module.exports = exports = { loadFunction };

test/fixtures/cjs-module/index.js

@@ -0,0 +1,3 @@
+module.exports = function testFunc(context) {
+  return context;
+};

test/fixtures/cjs-module/package.json

@@ -0,0 +1,4 @@
+{
+  "name": "cjs-module",
+  "version": "1.0.0"
+}

test/fixtures/esm-module-mjs/index.mjs

@@ -0,0 +1,21 @@
+const handle = async (context) => {
+  // YOUR CODE HERE
+  context.log.info(JSON.stringify(context, null, 2));
+
+  // If the request is an HTTP POST, the context will contain the request body
+  if (context.method === 'POST') {
+    return {
+      body: context.body,
+    }
+  // If the request is an HTTP GET, the context will include a query string, if it exists
+  } else if (context.method === 'GET') {
+    return {
+      query: context.query,
+    }
+  } else {
+    return { statusCode: 405, statusMessage: 'Method not allowed' };
+  }
+}
+
+// Export the function
+export { handle };

test/fixtures/esm-module-mjs/package.json

@@ -0,0 +1,4 @@
+{
+  "name": "esm-module-mjs",
+  "version": "1.0.0"
+}

test/fixtures/esm-module-type-module/index.js

@@ -0,0 +1,21 @@
+const handle = async context => {
+  // YOUR CODE HERE
+  context.log.info(JSON.stringify(context, null, 2));
+
+  // If the request is an HTTP POST, the context will contain the request body
+  if (context.method === 'POST') {
+    return {
+      body: context.body,
+    };
+  // If the request is an HTTP GET, the context will include a query string, if it exists
+  } else if (context.method === 'GET') {
+    return {
+      query: context.query,
+    };
+  } else {
+    return { statusCode: 405, statusMessage: 'Method not allowed' };
+  }
+};
+
+// Export the function
+export { handle };

test/fixtures/esm-module-type-module/package.json

@@ -0,0 +1,5 @@
+{
+  "name": "esm-module-type-module",
+  "version": "1.0.0",
+  "type": "module"
+}

test/test-function-loading.js

@@ -0,0 +1,23 @@
+const test = require('tape');
+const path = require('path');
+const { loadFunction } = require('../lib/function-loader.js');
+
+const fixtureDir = path.join(__dirname, 'fixtures');
+
+test('Loads a CJS function', async t => {
+  const fn = await loadFunction(path.join(fixtureDir, 'cjs-module', 'index.js'));
+  t.equal(typeof fn, 'function');
+  t.pass('CJS function loaded');
+});
+
+test('Loads an ESM function with an .mjs extension', async t => {
+  const fn = await loadFunction(path.join(fixtureDir, 'esm-module-mjs', 'index.mjs'));
+  t.equal(typeof fn.handle, 'function');
+  t.pass('ESM module with a mjs ext loaded');
+});
+
+test('Loads an ESM function with the type=module in the package.json', async t => {
+  const fn = await loadFunction(path.join(fixtureDir, 'esm-module-type-module', 'index.js'));
+  t.equal(typeof fn.handle, 'function');
+  t.pass('ESM module with a type=module in the package.json');
+});