Leetcode 6-z font conversion

/**
 * @author cookie
 * @param {string} s
 * @param {number} numRows
 * @return {string}
 */
var convert = function(s, numRows) {
    
    
    if(numRows<=1) return s;
    const length = s.length;
    const size = 2*numRows-2;
    let res = "";
    for(let j=0; j<=size/2; j++) {
    
    
        for(let i=0; i<=parseInt(length/size); i++) {
    
    
            let iCol1 = i*size + j;
            let iCol2 = (i+1)*size + -j;
            if(iCol1 < length) res += s[iCol1];
            if(j==0 || j==size/2) continue;
            if(iCol2 < length && iCol1!=iCol2) res += s[iCol2];
        }
    }
    return res;
};

const result = convert('abcdefghijklmnopqrstuvwxyz',3)
console.log(result);