function insertHyphen(str) { var strArr = str.split(''); var numArr = strArr.map(Number); for(var i = 0; i < numArr.length; i++) { if(numArr[i-1]%2===0 && numArr[i]%2===0) { numArr.push('-'); } } return numArr.join(''); } insertHyphen('112233445566'); // 112233445566---