[рони]

NovichokJS,

const splitText = (str, len = 10) => {
            if (typeof str !== 'string') {
                return null;
            }
            let txt = '';
            for (let i = 0; i < str.length; i++) {
                let k = i % len, e = str[i];
                txt += k == 0 ? e.toUpperCase() : k == len - 1 ? `${e}\n` : e
            }
            return txt;
        }
        console.log(splitText('abcdefghijklmnop', 4));
        /*  Abcd
            Efgh
            Ijkl
            Mnop
        */