feat: add transform function option for editing merged object

Added a new option `transform`, which takes a function that accepts the merged json object and
returns an object for output. Basically the same as `edit`, but instead of being applied file by
file, it's applied at the end.

README.md

@@ -17,6 +17,7 @@ gulp.src('jsonFiles/**/*.json')
 | ---- | ---- | ------- | ----------- |
 | `fileName` | `String` | `combined.json` | Output filename |
 | `edit` | `Function` | `json => json` | Edit function (add/remove/edit keys during merge) |
+| `transform` | `Function` | `json => json` | Transform final merged object (similar to edit but applied at the end) |
 | `startObj` | `Object/Array` | `{}` | Starting object to merge into (useful for providing default values) |
 | `endObj` | `Object/Array` | | Object to merge after file merging complete (useful for overwriting with special values) |
 | `exportModule` | `Boolean/String` | `false` | Output `module.exports = {MERGED_JSON_DATA};` or `{exportModule} = {MERGED_JSON_DATA}` when string passed |
@@ -54,6 +55,23 @@ gulp.src('jsonFiles/**/*.json')
 	}))
 	.pipe(gulp.dest('./dist'));
 
+/**
+ * Edit final JSON with transformer function
+ */
+gulp.src('jsonFiles/**/*.json')
+	.pipe(merge({
+		fileName: 'file.json',
+		transform: (mergedJson) => {
+			return {
+				key: {
+					type: 'data',
+					...mergedJson,
+				};
+			};
+		},
+	}))
+	.pipe(gulp.dest('./dist'));
+
 /**
  * Provide a default object (files are merged in order so object values will be overwritten)
  */

index.d.ts

@@ -12,6 +12,11 @@ declare module 'gulp-merge-json' {
      * @default json => json
      */
     edit?: (json: obj, file: File) => obj | void;
+    /**
+     * Transform function (edit final merged object)
+     * @default json => json
+     */
+    transform?: (json: obj) => obj;
     /**
      * Starting object to merge into (useful for providing default values)
      * @default {}

index.js

@@ -42,6 +42,7 @@ module.exports = function mergeJson(opts) {
     // Defaults
     fileName: 'combined.json',
     edit: (json) => json,
+    transform: (json) => json,
     startObj: {},
     endObj: null,
     exportModule: false,
@@ -101,7 +102,11 @@ module.exports = function mergeJson(opts) {
       merged = merge(merged, options.endObj, options);
     }
 
-    let contents = jsonLib.stringify(merged, options.jsonReplacer, options.jsonSpace);
+    let contents = jsonLib.stringify(
+      options.transform(merged),
+      options.jsonReplacer,
+      options.jsonSpace,
+    );
 
     if (options.exportModule === true) {
       contents = `module.exports = ${contents};`;

test/index.test.js

@@ -80,6 +80,23 @@ describe('gulp-merge-json', () => {
     });
   });
 
+  test('modified output based on transform function', (done) => {
+    const stream = gulp.src(['test/json/test1.json', 'test/json/test2.json']).pipe(merge({
+      transform: (json) => ({
+        name: json.name,
+        place: json.place,
+        tags: [...json.tags, 'sweet'],
+      }),
+    }));
+
+    stream.on('data', (file) => {
+      const expected = ['{', '\t"name": "Josh",', '\t"place": "San Francisco",', '\t"tags": [', '\t\t"awesome",', '\t\t"fun",', '\t\t"sweet"', '\t]', '}'].join('\n');
+
+      expect(file.contents.toString()).toBe(expected);
+      done();
+    });
+  });
+
   test('uses supplied start object as base', (done) => {
     const stream = gulp.src(['test/json/test1.json', 'test/json/test2.json']).pipe(merge({
       startObj: { initial: 'value' },