Add source map helper

Summary: Adds a helper to produce index source maps for Buck builds

Reviewed By: cpojer

Differential Revision: D4265911

fbshipit-source-id: 9ca3c49876df5db039bae823c0458c98e6e05619

react-packager/src/ModuleGraph/output/source-map.js

@@ -0,0 +1,48 @@
+/**
+ * Copyright (c) 2016-present, Facebook, Inc.
+ * All rights reserved.
+ *
+ * This source code is licensed under the BSD-style license found in the
+ * LICENSE file in the root directory of this source tree. An additional grant
+ * of patent rights can be found in the PATENTS file in the same directory.
+ *
+ * @flow
+ */
+'use strict';
+
+type CreateIndexMapOptions = {|
+  file?: string,
+  sections?: Array<IndexMapSection>
+|};
+
+type IndexMap = MapBase & {
+  sections: Array<IndexMapSection>,
+};
+
+type IndexMapSection = {
+  map: IndexMap | MappingsMap,
+  offset: {line: number, column: number},
+};
+
+type MapBase = {
+  // always the first entry in the source map entry object per
+  // https://fburl.com/source-map-spec#heading=h.qz3o9nc69um5
+  version: 3,
+  file?: string,
+};
+
+type MappingsMap = MapBase & {
+  mappings: string,
+  names: Array<string>,
+  sourceRoot?: string,
+  sources: Array<string>,
+  sourcesContent?: Array<?string>,
+};
+
+export type SourceMap = IndexMap | MappingsMap;
+
+exports.createIndexMap = (opts?: CreateIndexMapOptions): IndexMap => ({
+  version: 3,
+  file: opts && opts.file,
+  sections: opts && opts.sections || [],
+});