S.js

// you can write to stdout for debugging purposes, e.g.
// console.log('this is a debug message');

function solution(S) {
    // write your code in JavaScript (Node.js 8.9.4)

    let maxLen = -1;
    let currentLen = 0;

    let charCount = 0;
    let numbCount = 0;
    let err=0;

    for(let i=0; i<=S.length; i++) {
        if(i==S.length || S[i]==' ') {
            if(err==0 
                && (numbCount&1 || numbCount==0) 
                && (!(charCount&1) || charCount==0)
                && currentLen>maxLen) {
                maxLen=currentLen;
            }
            err=0;
            numbCount=0;
            charCount=0;
            currentLen=0;
            continue;
        } else if (/^[a-zA-Z]+$/i.test(S[i])) {
            charCount++;
        } else if (/^[0-9]+$/i.test(S[i])) {
            numbCount++;
        } else {
            err=1;
        }
        currentLen++;

        // console.log(numbCount, charCount, currentLen, err, maxLen);
    }

    return maxLen;
}