A simple JavaScript implementation of Reservoir Sampling with any K length (http://www.geeksforgeeks.org/reservoir-sampling/)

reservoir_sampling.js

class Node {
    constructor(value, next) {
        this.value = value;
        this.next = next;
    }
}

function getRandom(list, k) {
    const reservoir = new Array(k);

    for (let i = 0, ptr = list; ptr != undefined; i++, ptr = ptr.next) {
        stream(reservoir, i, ptr);
    }

    return reservoir;
}

function stream(reservoir, n, input) {
    if (n < reservoir.length) { // If we're still below the reservoir's threshold, then automatically put it in
        reservoir[n] = input;
    } else {
        let r = randomIntInRange(0, n, true); // Generate a random number from 0 (inclusive) to N (inclusive)
        if (r < reservoir.length) { // If the random number is within the reservoir's threshold range,
            reservoir[r] = input;
        }
    }
}

function randomIntInRange(min, max, inclusive) {
    const range = (max - min) + ((inclusive) ? 1 : 0);
    return Math.trunc(Math.random() * range + min);
}

const linkedList = new Node('a',
                        new Node('b',
                            new Node('c',
                                new Node('d',
                                    new Node('e')))));

const randomNode = getRandom(linkedList, 1); // We're setting K = 1, here since we're only picking a single node. But K can be higher for larger lists or streams.
console.log(`The randomly selected node is ${randomNode[0].value}`);