Array.prototype.unique = function () {
    let x,
        y;
    let uni = {};
    for (x of this) {
        uni[x] = 'xfy';
    }
    let arr = Object.keys(uni);
    return arr;
}

Array.prototype.unique = function () {
    let temp = {},
        arr = [];
    let len = this.length;
    for (let i = 0; i < len; i++) {
        if(!temp[this[i]]) {
            temp[this[i]] = 'xfy';
            arr.push(this[i]);
        }
    }
    return arr;
}

// 不完善
Array.prototype.unique = function () {
    let arr = [],
        x,
        y;
    for (x of this) {
        if (y === x) continue;
        arr.push(x);
        y = x;
    }
    return arr;
}
let arr = [1, 2, 2, 2, 3, 3, 4, 5, 6, 6, 7, 8, 9, 9, 'a', 'a', 'a', 'b', 'b', 'b', 'c'];
arr.unique();