Fix validation error messages in the browser (#9073)

We converted the ParsingError and ValidationError classes to inherit from the built-in Error class in #9025. This works fine in ES6, so our unit tests were happy. However, transpiling it down to ES5 leads to an absence of the message property, essentially removing the text of parsing and validation errors.

src/source/video_source.js

@@ -119,7 +119,7 @@ class VideoSource extends ImageSource {
         if (this.video) {
             const seekableRange = this.video.seekable;
             if (seconds < seekableRange.start(0) || seconds > seekableRange.end(0)) {
-                this.fire(new ErrorEvent(new ValidationError(`Playback for this video can be set only between the ${seekableRange.start(0)} and ${seekableRange.end(0)}-second mark.`)));
+                this.fire(new ErrorEvent(new ValidationError(`sources.${this.id}`, null, `Playback for this video can be set only between the ${seekableRange.start(0)} and ${seekableRange.end(0)}-second mark.`)));
             } else this.video.currentTime = seconds;
         }
     }

src/style-spec/error/parsing_error.js

@@ -1,12 +1,15 @@
 // @flow
 
-export default class ParsingError extends Error {
+// Note: Do not inherit from Error. It breaks when transpiling to ES5.
+
+export default class ParsingError {
+    message: string;
     error: Error;
     line: number;
 
     constructor(error: Error) {
-        super(error.message);
         this.error = error;
+        this.message = error.message;
         const match = error.message.match(/line (\d+)/);
         this.line = match ? parseInt(match[1], 10) : 0;
     }

src/style-spec/error/validation_error.js

@@ -1,11 +1,14 @@
 // @flow
 
-export default class ValidationError extends Error {
+// Note: Do not inherit from Error. It breaks when transpiling to ES5.
+
+export default class ValidationError {
+    message: string;
     identifier: ?string;
     line: ?number;
 
-    constructor(key: string | null, value?: any, message?: string, identifier?: string) {
-        super([key, message].filter(a => a).join(': '));
+    constructor(key: ?string, value: ?{ __line__: number }, message: string, identifier: ?string) {
+        this.message = (key ? `${key}: ` : '') + message;
         if (identifier) this.identifier = identifier;
 
         if (value !== null && value !== undefined && value.__line__) {

src/util/evented.js

@@ -31,10 +31,14 @@ export class Event {
     }
 }
 
+interface ErrorLike {
+    message: string;
+}
+
 export class ErrorEvent extends Event {
-    error: Error;
+    error: ErrorLike;
 
-    constructor(error: Error, data: Object = {}) {
+    constructor(error: ErrorLike, data: Object = {}) {
         super('error', extend({error}, data));
     }
 }

test/unit/style-spec/validate.test.js

@@ -6,23 +6,14 @@ import validate from '../../../src/style-spec/validate_style';
 
 const UPDATE = !!process.env.UPDATE;
 
-function sanitizeError(error) {
-    const sanitized = {};
-    sanitized.message = error.message;
-    for (const key in error) {
-        sanitized[key] = error[key];
-    }
-    return sanitized;
-}
-
 glob.sync(`${__dirname}/fixture/*.input.json`).forEach((file) => {
     test(path.basename(file), (t) => {
         const outputfile = file.replace('.input', '.output');
         const style = fs.readFileSync(file);
         const result = validate(style);
         if (UPDATE) fs.writeFileSync(outputfile, JSON.stringify(result, null, 2));
         const expect = JSON.parse(fs.readFileSync(outputfile));
-        t.deepEqual(result.map(sanitizeError), expect);
+        t.deepEqual(result, expect);
         t.end();
     });
 });

test/unit/style-spec/validate_mapbox_api_supported.test.js

@@ -6,23 +6,14 @@ import validateMapboxApiSupported from '../../../src/style-spec/validate_mapbox_
 
 const UPDATE = !!process.env.UPDATE;
 
-function sanitizeError(error) {
-    const sanitized = {};
-    sanitized.message = error.message;
-    for (const key in error) {
-        sanitized[key] = error[key];
-    }
-    return sanitized;
-}
-
 glob.sync(`${__dirname}/fixture/*.input.json`).forEach((file) => {
     test(path.basename(file), (t) => {
         const outputfile = file.replace('.input', '.output-api-supported');
         const style = fs.readFileSync(file);
         const result = validateMapboxApiSupported(style);
         if (UPDATE) fs.writeFileSync(outputfile, JSON.stringify(result, null, 2));
         const expect = JSON.parse(fs.readFileSync(outputfile));
-        t.deepEqual(result.map(sanitizeError), expect);
+        t.deepEqual(result, expect);
         t.end();
     });
 });