startsWith和endsWith

if (typeof String.prototype.startsWith != 'function') {
  String.prototype.startsWith = function (prefix){
    return this.slice(0, prefix.length) === prefix;
  };
}

if (typeof String.prototype.endsWith != 'function') {
  String.prototype.endsWith = function(suffix) {
    return this.indexOf(suffix, this.length - suffix.length) !== -1;
  };
}