JavaScript数组去重的4种常用实现方法

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Array.prototype.indexOf = Array.prototype.indexOf || function(item) {
  for (var i = 0, j = this.length; i < j; i++) {
    if (this[i] === item) {
      return i;
    }
  }
  return -1;
}
Array.prototype.forEach = Array.prototype.forEach || function(callback, thisArg) {
  if (!callback || typeof callback !== 'function') return;
  for (var i = 0, j = this.length; i < j; i++) {
    callback.call(thisArg, this[i], i, this);
  }
}

function removeDuplicatedItem(ar) {
  var ret = [];
  for (var i = 0, j = ar.length; i < j; i++) {
    if (ret.indexOf(ar[i]) === -1) {
      ret.push(ar[i]);
    }
  }
  return ret;
}

function removeDuplicatedItem2(ar) {
  var tmp = {},
    ret = [];
  for (var i = 0, j = ar.length; i < j; i++) {
    if (!tmp[ar[i]]) {
      tmp[ar[i]] = 1;
      ret.push(ar[i]);
    }
  }
  return ret;
}

function removeDuplicatedItem3(ar) {
  var ret = [];
  ar.forEach(function(e, i, ar) {
    if (ar.indexOf(e) === i) {
      ret.push(e);
    }
  });
  return ret;
}

function removeDuplicatedItem4(ar) {
  var ret = [],
    end;
  ar.sort();
  end = ar[0];
  ret.push(ar[0]);
  for (var i = 1; i < ar.length; i++) {
    if (ar[i] != end) {
      ret.push(ar[i]);
      end = ar[i];
    }
  }
  return ret;
}