nani.js

var balancedString = function(s) {
    const n4 = s.length / 4;
    s = s.split('');
    
    const balance = {
        Q: 0,
        W: 0,
        E: 0,
        R: 0,
    }
    
    s.forEach(c => {
        balance[c] += 1; 
    });

    let return_n = false;
    
    Object.keys(balance).forEach(key => {
        if (balance[key] === s.length) {
            return_n = true;
        }
        
        balance[key] -= n4;
        balance[key] *= -1;
    });
    
    if (isBalanced(balance)) {
        return 0;
    }
    
    if (return_n) { return s.length * 3 / 4; }
        
    let current = {
        Q: balance.Q,
        W: balance.W,
        E: balance.E,
        R: balance.R,
    };
    let start = 0;
    let end = 0;
    
    let min = Infinity;
        
    while (end < s.length) {        
        current[s[end]] += 1;
        end += 1;
        
        if (isBalanced(current)) {
            while (start < end) {                
                if (!isBalanced(current)) {
                    end = start;
                    current = {
                        Q: balance.Q,
                        W: balance.W,
                        E: balance.E,
                        R: balance.R,
                    };
                    break;
                }
                
                min = Math.min(min, end - start);
                
                current[s[start]] -= 1;
                start += 1;
            }
        }
        

    }
    
    return min;
};

function isBalanced(balance) {
    return balance.Q >= 0 && balance.W >= 0 && balance.E >= 0 && balance.R >= 0;
}