src/URIUtils.js

// Prefix of data URIs emitted by SINGLE_FILE and related options.
var dataURIPrefix = 'data:application/octet-stream;base64,';

// Indicates whether filename is a base64 data URI.
function isDataURI(filename) {
  return String.prototype.startsWith ?
      filename.startsWith(dataURIPrefix) :
      filename.indexOf(dataURIPrefix) === 0;
}