function merge(oldMap, newMap) {
  if (!oldMap) return newMap
  if (!newMap) return oldMap

  var oldMapConsumer = new SourceMapConsumer(oldMap);
  var newMapConsumer = new SourceMapConsumer(newMap);
  var mergedMapGenerator = new SourceMapGenerator();

  // iterate on new map and overwrite original position of new map with one of old map
  newMapConsumer.eachMapping(function(m) {
    // pass when `originalLine` is null.
    // It occurs in case that the node does not have origin in original code.
    if (m.originalLine == null) return

    var origPosInOldMap = oldMapConsumer.originalPositionFor({
      line: m.originalLine,
      column: m.originalColumn
    });

    if (origPosInOldMap.source == null) return

    mergedMapGenerator.addMapping({
      original: {
        line: origPosInOldMap.line,
        column: origPosInOldMap.column
      },
      generated: {
        line: m.generatedLine,
        column: m.generatedColumn
      },
      source: origPosInOldMap.source,
      name: origPosInOldMap.name
    });
  });

  var consumers = [oldMapConsumer, newMapConsumer];
  consumers.forEach(function(consumer) {
    consumer.sources.forEach(function(sourceFile) {
      mergedMapGenerator._sources.add(sourceFile);
      var sourceContent = consumer.sourceContentFor(sourceFile);
      if (sourceContent != null) {
        mergedMapGenerator.setSourceContent(sourceFile, sourceContent);
      }
    });
  });

  mergedMapGenerator._sourceRoot = oldMap.sourceRoot;
  mergedMapGenerator._file = oldMap.file;

  return JSON.parse(mergedMapGenerator.toString())
}
Merge old source map and new source map and return merged.
If old or new source map value is falsy, return another one as it is.

@param {object|string} [oldMap] old source map object
@param {object|string} [newmap] new source map object
@return {object|undefined} merged source map object, or undefined when both old and new source map are undefined