function separate(str) {
    let result = [];
    let regEx = /[^a-z]/
        
    str = str.toLowerCase();
    
    for(let i = 0; i < str.length-1; i++) {
        if(regEx.test(str.slice(i,i+2))) continue;
        result.push(str.slice(i,i+2));
    }
    return result;
}

function solution(str1, str2) {
    str1 = separate(str1);
    str2 = separate(str2);
    if(!str1.length && !str2.length) return 65536;
    
    let union = [];
    let intersection = [];
    
    for(let i = 0; i<str2.length; i++) {
        if(str1.includes(str2[i])) {
            intersection.push(...str1.splice(str1.indexOf(str2[i]),1));
        }
    union.push(str2[i]);
        
    }
    union.push(...str1)
    return parseInt(intersection.length / union.length * 65536);
}

다중집합에 대해 교집합 합집합을 구하는 방법

let union = [];
let intersection = [];

for(let i = 0; i<arr1.length; i++) {
    if(arr2.includes(arr1[i])) {
        intersection.push(arr2.splice(arr2.indexOf(arr1[i]),1))
    }
    union.push(arr1[i]);
}
union.push(...arr2);

splice가 핵심이다.

참고

https://velog.io/@sso/%ED%94%84%EB%A1%9C%EA%B7%B8%EB%9E%98%EB%A8%B8%EC%8A%A4-javascript-1%EC%B0%A8%EB%89%B4%EC%8A%A4%ED%81%B4%EB%9F%AC%EC%8A%A4%ED%84%B0%EB%A7%81