fix: 🐛 issue when notifyOnChangeOnly stops emitting at all

When a user provides notifyOnChangeOnly option, we are trying to compare
the new metadata with the old one by stringifying them. Turns out, that
JSON.stringify() does not work on Map, so it returns an empty object all
the time. Leading to broken comparsion that says that metadata never
changes.

.eslintrc.js

@@ -49,6 +49,7 @@ module.exports = {
     'max-len': ['error', 120],
     'max-lines-per-function': ['off'],
     'max-statements': ['off'],
+    'multiline-comment-style': ['error', 'separate-lines'],
     'multiline-ternary': ['error', 'always-multiline'],
     'no-ternary': ['off'],
     'node/no-missing-import': ['error', { tryExtensions: ['.ts'] }],

examples/options.ts

@@ -6,7 +6,7 @@ const radioStation = new Parser({
   errorInterval: 10 * 60,
   keepListen: false,
   metadataInterval: 5,
-  notifyOnChangeOnly: false,
+  notifyOnChangeOnly: true,
   url: 'https://live.hunter.fm/80s_high',
   userAgent: 'Custom User Agent',
 });

src/Parser.ts

@@ -28,7 +28,7 @@ export declare interface Parser {
 }
 
 export class Parser extends EventEmitter {
-  private previousMetadata = '';
+  private previousMetadata: Map<string, string> = new Map<string, string>();
   private readonly options: ParserOptions = {
     autoUpdate: true,
     emptyInterval: 5 * 60,
@@ -61,10 +61,9 @@ export class Parser extends EventEmitter {
         this.destroyResponse(response);
         this.queueNextRequest(this.options.metadataInterval);
 
-        const newMetadata = JSON.stringify(metadata);
-        if (this.options.notifyOnChangeOnly && newMetadata !== this.previousMetadata) {
+        if (this.options.notifyOnChangeOnly && this.isMetadataChanged(metadata)) {
+          this.previousMetadata = metadata;
           this.emit('metadata', metadata);
-          this.previousMetadata = newMetadata;
         } else if (!this.options.notifyOnChangeOnly) {
           this.emit('metadata', metadata);
         }
@@ -114,4 +113,14 @@ export class Parser extends EventEmitter {
   protected queueRequest (timeout = 0): void {
     setTimeout(this.makeRequest.bind(this), timeout * 1000);
   }
+
+  protected isMetadataChanged (metadata: Map<string, string>): boolean {
+    for (const [key, value] of metadata.entries()) {
+      if (this.previousMetadata.get(key) !== value) {
+        return true;
+      }
+    }
+
+    return false;
+  }
 }

test/Parser.spec.ts

@@ -53,4 +53,15 @@ describe('parser', () => {
       resolve();
     });
   }));
+
+  it('should properly emit metadata event when metadata has been updated', async () => await new Promise((resolve) => {
+    expect.hasAssertions();
+
+    const radio = new Parser({ autoUpdate: false, notifyOnChangeOnly: true, url: 'https://live.hunter.fm/80s_high' });
+    radio.on('metadata', (metadata) => {
+      // @ts-expect-error I want to check that metadata was stored in the private property to later comparsion
+      expect(radio.previousMetadata).toStrictEqual(metadata);
+      resolve();
+    });
+  }));
 });