Refactor mergeSourceMaps to its own file

Reviewed By: davidaurelio

Differential Revision: D10488231

fbshipit-source-id: 62be8989cbc31c3a824bb460b74dfb12b12a6831

packages/metro/src/ModuleGraph/worker/mergeSourceMaps.js

@@ -0,0 +1,47 @@
+
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @format
+ * @flow strict-local
+ */
+
+'use strict';
+
+// eslint-disable-next-line lint/flow-no-fixme
+// $FlowFixMe: too hard to type, and they only have a .ts file.
+const sourceMap = require('source-map');
+
+import type {BabelSourceMap} from '@babel/core';
+import type {MetroSourceMap} from 'metro-source-map';
+
+function mergeSourceMaps(
+  file: string,
+  originalMap: MetroSourceMap,
+  secondMap: MetroSourceMap,
+): BabelSourceMap {
+  const merged = new sourceMap.SourceMapGenerator();
+  const inputMap = new sourceMap.SourceMapConsumer(originalMap);
+  new sourceMap.SourceMapConsumer(secondMap).eachMapping(mapping => {
+    const original = inputMap.originalPositionFor({
+      line: mapping.originalLine,
+      column: mapping.originalColumn,
+    });
+    if (original.line == null) {
+      return;
+    }
+
+    merged.addMapping({
+      generated: {line: mapping.generatedLine, column: mapping.generatedColumn},
+      original: {line: original.line, column: original.column || 0},
+      source: file,
+      name: original.name || mapping.name,
+    });
+  });
+  return merged.toJSON();
+}
+
+module.exports = mergeSourceMaps;