feat: add --esm option for ley new command (#25)

* Add --mjs option to generate ESM migration

* Apply suggestions from code review

* Update index.js

* add `opts.esm` docs to readme

* update typedefs

* add `--esm` mention to ESM docs section

* Update index.js

Co-authored-by: Luke Edwards <[email protected]>

Author: karlhorky

index.js

@@ -102,15 +102,21 @@ exports.new = async function (opts={}) {
 	}
 
 	let filename = prefix + '-' + opts.filename.replace(/\s+/g, '-');
-	if (!/\.\w+$/.test(filename)) filename += '.js';
+	if (!/\.\w+$/.test(filename)) filename += opts.esm ? '.mjs' : '.js';
 	let dir = resolve(opts.cwd || '.', opts.dir);
 	let file = join(dir, filename);
 
-	await mkdir(dir).then(() => {
-		let str = 'exports.up = async client => {\n\t// <insert magic here>\n};\n\n';
-		str += 'exports.down = async client => {\n\t// just in case...\n};\n';
-		writeFileSync(file, str);
-	});
+	let str = '';
+	await mkdir(dir);
+	
+	if (opts.mjs) {
+		str += 'export async function up(client) {\n\n}\n\n';
+		str += 'export async function down(client) {\n\n}\n';
+	} else {
+		str += 'exports.up = async client => {\n\n};\n\n';
+		str += 'exports.down = async client => {\n\n};\n';
+	}
+	writeFileSync(file, str);
 
 	return filename;
 }

ley.d.ts

@@ -22,6 +22,7 @@ declare namespace Options {
 		filename: string;
 		timestamp?: boolean;
 		length?: number;
+		esm?: boolean;
 	}
 }
 

readme.md

@@ -241,6 +241,20 @@ export async function down(DB) {
 }
 ```
 
+You may generate new migration files in ESM syntax by passing the `--esm` flag to the `ley new` command:
+
+```sh
+$ ley new todos --esm
+#=> migrations/003-todos.mjs
+
+$ cat migrations/003-todos.mjs
+#=> export async function up(client) {
+#=> }
+#=> 
+#=> export async function down(client) {
+#=> }
+```
+
 ## Drivers
 
 Out of the box, `ley` includes drivers for the following npm packages:
@@ -339,7 +353,15 @@ Type: `string`
 
 **Required.** The name of the file to be created.
 
-> **Note:** A prefix will be prepended based on [`opts.timestamp`](#optstimestamp) and [`opts.length`](#optslength) values.<br>The `.js` extension will be applied unless your input already has an extension.
+> **Note:** A prefix will be prepended based on [`opts.timestamp`](#optstimestamp) and [`opts.length`](#optslength) values.<br>If your input does not already end with an extension, then `.js` or `.mjs` will be appended.
+
+#### opts.esm
+Type: `boolean`<br>
+Default: `false`
+
+Create a migration file with ESM syntax.
+
+> **Note:** When true, the `opts.filename` will contain the `.mjs` file extension unless your input already has an extension.
 
 #### opts.timestamp
 Type: `boolean`<br>