Javascript中实现String.startsWith和endsWith方法

"abcd".startsWith("ab"); // true
"abcd".startsWith("bc"); // false
"abcd".endsWith("cd");  // true
"abcd".endsWith("e");  // false
"a".startsWith("a");   // true
"a".endsWith("a");    // true

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;
 };
}