core-js/modules/$.string-context.js

Standard library

// helper for String#{startsWith, endsWith, includes}
var defined = require('./$.defined')
  , cof     = require('./$.cof');

module.exports = function(that, searchString, NAME){
  if(cof(searchString) == 'RegExp')throw TypeError('String#' + NAME + " doesn't accept regex!");
  return String(defined(that));
};