uniq.js

// http://qiita.com/bellbind/items/50954258e262fd7c6bd1

function uniq1(A, i = 0, r = [], h = A[i], l = r.length) {
    if (i === A.length) return r;
    return uniq1(A, i + 1, l === 0 ? [h] : h === r[l - 1] ? r : [...r, h]);
}


function uniq2(array) {
    const result = [];

    for (let i = 0; i < array.length; i++) {
        if (i === 0 || array[i - 1] !== array[i]) {
            result.push(array[i]);
        }
    }

    return result;
}

function uniq3(A) {
    let r = [];

    for (let i = 0; i < A.length; i++) {
        const l = r.length;
        const h = A[i];
        if (l === 0) {
            r = [h];
        } else if (r[l - 1] !== h) {
            r = [...r, h];
        }
    }

    return r;
}

(function() {
    var array = [];
    for (let i = 0; i < 1000; i++) {
        array.push((Math.random() * 100)|0);
    }
    array.sort();

    const start = new Date().getTime();

    let sum = 0;

    for (let i = 0; i < 100; i++) {
        sum += uniq3(array).length;
    }

    const end = new Date().getTime();

    console.log(sum, end - start);
})();

console.log(uniq1([1, 2, 2, 3, 3, 3, 4, 4, 4, 4])); //=> [1, 2, 3, 4]
console.log(uniq2([1, 2, 2, 3, 3, 3, 4, 4, 4, 4])); //=> [1, 2, 3, 4]

console.log(uniq1([undefined, undefined, undefined])); //=> [undefined]
console.log(uniq2([undefined, undefined, undefined])); //=> [undefined]

// Firefox 48.0.2
// uniq2: 2772 ms
// uniq3:  204 ms