1. 通过ES6新增数据类型Set的特性

Array.prototype.unique = function() {
  return [...new Set(this)];
};

2. 通过for循环+indexOf过滤，并用新开辟的存储空间保存合适的数组元素

Array.prototype.unique = function() {
  const result = [];
  for (let i = 0, len = this.length; i < len; i++) {
    // 或 result.includes(this[i]) --> return true/false
    if (result.indexOf(this[i]) === -1) result.push(this[i]);
  }

  return result;
};

3. 借助hash算法的思想

Array.prototype.unique = function() {
  const record = {};
  const result = [];
  const len = this.length;
  let curr, i;

  for (i = 0; i < len; i++) {
    curr = this[i];
    if (record[curr]) continue;
    record[curr] = true;
    result.push(curr);
  }

  return result;
};