fix: using StringDecoder to decode fit strings

src/fit2json.js

@@ -1,4 +1,12 @@
 const constants = require('./constants');
+const { StringDecoder } = require('node:string_decoder');
+
+/**
+ * Parses the buffer as a UTF-8 string.
+ */
+function decodeString(buffer) {
+  return new StringDecoder('utf8').end(Buffer.from(buffer));
+}
 
 /**
  * Parses one record of a FIT file
@@ -169,7 +177,7 @@ function readDataField (baseType, fit, pointer, size, littleEndian) {
           result.push(fit.getUint8(pointer + i));
         }
       }
-      return String.fromCharCode(...result);
+      return decodeString(result);
   }
 }
 

test/fit2json.spec.js

@@ -116,6 +116,13 @@ describe('/src/fit2json', () => {
       const result = fit2json.readDataField(0x07, fit, 0, 6, false);
       expect(result).to.equal('string');
     });
+
+    it('should decode the string base type (0x07) as UTF-8', () => {
+      const fit = new DataView(new Uint8Array([0xf0, 0x9f, 0x92, 0xaa]).buffer);
+      const result = fit2json.readDataField(0x07, fit, 0, 4, false);
+      expect(result).to.equal('\uD83D\uDCAA');
+    });
+
     it('should read a byte base type (0x0D) in big endian notation', () => {
       const fit = new DataView(new Uint8Array([115,116,114,105,110,103]).buffer);
       const result = fit2json.readDataField(0x0D, fit, 0, 6, false);