Ensure source map can be resolved from cached code

lib/caching-precompiler.js

@@ -4,6 +4,7 @@ var cachingTransform = require('caching-transform');
 var md5Hex = require('md5-hex');
 var stripBom = require('strip-bom');
 var objectAssign = require('object-assign');
+var generateMapFileComment = require('convert-source-map').generateMapFileComment;
 
 module.exports = CachingPrecompiler;
 
@@ -59,9 +60,22 @@ CachingPrecompiler.prototype._factory = function (babelConfig, cacheDir) {
 		code = code.toString();
 		var options = buildOptions(filename, code);
 		var result = babel.transform(code, options);
-		var mapFile = path.join(cacheDir, hash + '.map');
+		var mapFile = path.join(cacheDir, hash + '.js.map');
 		fs.writeFileSync(mapFile, JSON.stringify(result.map));
-		return result.code;
+
+		// When loading the test file, test workers intercept the require call and
+		// load the cached code instead. Libraries like nyc may also be intercepting
+		// require calls, however they won't know that different code was loaded.
+		// They may then attempt to resolve a source map from the original file
+		// location.
+		//
+		// Add a source map file comment to the cached code. The file path is
+		// relative from the directory of the original file to where the source map
+		// is cached. This will allow the source map to be resolved.
+		var sourceDir = path.dirname(filename);
+		var relMapFile = path.relative(sourceDir, mapFile);
+		var mapFileComment = generateMapFileComment(relMapFile);
+		return result.code + '\n' + mapFileComment;
 	};
 };
 

lib/test-worker.js

@@ -72,7 +72,7 @@ installPrecompiler(function (filename) {
 	var precompiled = opts.precompiled[filename];
 
 	if (precompiled) {
-		sourceMapCache[filename] = path.join(cacheDir, precompiled + '.map');
+		sourceMapCache[filename] = path.join(cacheDir, precompiled + '.js.map');
 		return fs.readFileSync(path.join(cacheDir, precompiled + '.js'), 'utf8');
 	}
 

test/caching-precompiler.js

@@ -6,6 +6,7 @@ var uniqueTempDir = require('unique-temp-dir');
 var sinon = require('sinon');
 var babel = require('babel-core');
 var transformRuntime = require('babel-plugin-transform-runtime');
+var fromMapFileSource = require('convert-source-map').fromMapFileSource;
 
 var CachingPrecompiler = require('../lib/caching-precompiler');
 
@@ -18,7 +19,7 @@ function endsWithJs(filename) {
 }
 
 function endsWithMap(filename) {
-	return /\.js$/.test(filename);
+	return /\.js\.map$/.test(filename);
 }
 
 sinon.spy(babel, 'transform');
@@ -50,7 +51,35 @@ test('adds files and source maps to the cache directory as needed', function (t)
 	var files = fs.readdirSync(tempDir);
 	t.is(files.length, 2);
 	t.is(files.filter(endsWithJs).length, 1, 'one .js file is saved to the cache');
-	t.is(files.filter(endsWithMap).length, 1, 'one .map file is saved to the cache');
+	t.is(files.filter(endsWithMap).length, 1, 'one .js.map file is saved to the cache');
+	t.end();
+});
+
+test('adds a map file comment to the cached files', function (t) {
+	var tempDir = uniqueTempDir();
+	var precompiler = new CachingPrecompiler(tempDir, null);
+
+	precompiler.precompileFile(fixture('es2015.js'));
+
+	var cachedCode;
+	var cachedMap;
+	fs.readdirSync(tempDir).map(function (file) {
+		return path.join(tempDir, file);
+	}).forEach(function (file) {
+		if (endsWithJs(file)) {
+			cachedCode = fs.readFileSync(file, 'utf8');
+		} else if (endsWithMap(file)) {
+			cachedMap = fs.readFileSync(file, 'utf8');
+		}
+	});
+
+	// This is comparable to how nyc resolves the source map. It has access to the
+	// cached code but believes it to come from the original es2015.js fixture.
+	// Ensure the cached map can be resolved from the cached code. Also see
+	// <https://github.com/bcoe/nyc/blob/69ed03b29c423c0fd7bd41f9dc8e7a3a68f7fe50/index.js#L244>.
+	var foundMap = fromMapFileSource(cachedCode, path.join(__dirname, 'fixture'));
+	t.ok(foundMap);
+	t.is(foundMap.toJSON(), cachedMap);
 	t.end();
 });