code: function createSetHeader (options) {
    // response time digits
    var digits = options.digits !== undefined
      ? options.digits
      : 3
  
    // header name
    var header = options.header || 'X-Response-Time'
  
    // display suffix
    var suffix = options.suffix !== undefined
      ? Boolean(options.suffix)
      : true
  
    return function setResponseHeader (req, res, time) {
      if (res.getHeader(header)) {
        return
      }
  
      var val = time.toFixed(digits)
  
      if (suffix) {
        val += 'ms'
      }
  
      res.setHeader(header, val)
    }
  }
docs: Create function to set respoonse time header.
  @private