code: 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())
  }
docs: 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