ensure source map can be resolved from cached 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.

This commit adds 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.

Add a test which mimics how nyc resolves the source map.

Author: novemberborn

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;
 
@@ -61,7 +62,20 @@ CachingPrecompiler.prototype._factory = function (babelConfig, cacheDir) {
 		var result = babel.transform(code, options);
 		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;
 	};
 };
 

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');
 
@@ -54,6 +55,34 @@ test('adds files and source maps to the cache directory as needed', function (t)
 	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();
+});
+
 test('uses default babel options when babelConfig === "default"', function (t) {
 	var tempDir = uniqueTempDir();
 	var precompiler = new CachingPrecompiler(tempDir, 'default');