[рони]

NovichokJS,

function numberOfPairs(gloves) {
    const obj = {};
    let count = 0;
    for (let el of gloves) {
       obj[el] = (obj[el]||0)+1;
       obj[el] %= 2;
       obj[el] || count++;
    }
    return count;
}

console.log(numberOfPairs(["red", "green", "red", "blue", "blue"])); // 2
console.log(numberOfPairs(["gray", "green", "red", "blue", "yellow"])); // 0
console.log(numberOfPairs(["gray", "green", "red", "green", "yellow"])); // 1
console.log(numberOfPairs(["red", "red", "yellow"])); // 1
console.log(numberOfPairs(["red", "red", "red", "red", "red", "red"])); // 3