sushi.js

function getMaxEatenDiscountCount_foreach(N, D, K) {
    let eaten = new Set();
    let eatSum = 0;
    let eatCounter = 0;
    D.forEach((dish) => {
        if (!eaten.has(dish)) {
            eatSum++;
            eaten.add(dish);
            eatCounter++;
        }
        if (eatCounter > K) {
            eaten.delete(eaten.values().next().value);
            eatCounter--;
        }
    });
    return eatSum;
}

function getMaxEatenDiscountCount_forloop(N, D, K) {
    let eaten = new Set();
    let eatSum = 0;
    let eatCounter = 0;
    for (dish in D) {
        if (!eaten.has(dish)) {
            eatSum++;
            eaten.add(dish);
            eatCounter++;
        }
        if (eatCounter > K) {
            eaten.delete(eaten.values().next().value);
            eatCounter--;
        }
    };
    return eatSum;
}

function randomInt(N) {
    return Math.floor(Math.random() * N);
}

function randomIntArray(N, size) {
    return Array.from({length:size}, (_,i) => randomInt(N));
}

let N = 50000;
let MaxD = 100000;
let D = randomIntArray(MaxD, N);
let K = 25000;

console.time('getMaxEatenDiscountCount_foreach');
getMaxEatenDiscountCount_foreach(N, D, K);
console.timeEnd('getMaxEatenDiscountCount_foreach');

console.time('getMaxEatenDiscountCount_forloop');
getMaxEatenDiscountCount_forloop(N, D, K);
console.timeEnd('getMaxEatenDiscountCount_forloop');

Result:

$ node /tmp/sushi.js 
getMaxEatenDiscountCount_foreach: 138.572ms
getMaxEatenDiscountCount_forloop: 305.100ms