function sep(letters){
if(!letters) return [];
let arrLetters = letters.split('')
let arrPair = []
for(i=0;i<arrLetters.length;i+=2){
if(!arrLetters[i+1]) arrLetters[i+1] = '_';
alert()
arrPair.push(arrLetters[i]+arrLetters[i+1])
}
return arrPair
}
sep('abc') // ['ab', 'c_']
sep('abcdef') // ['ab', 'cd', 'ef']