[Дмитрий Луценко]

const splitText = (str, len) => {
    const strArr = [];
    let i = 0;
    if (typeof str !== 'string') { return null; }

    while (i < str.length) {

        let chunk = str.substring(i, i + len);

        strArr.push(chunk[0].toUpperCase() + chunk.slice(1));

        strArr.join('\n');
        i += (len);
    }
    return strArr;
    
}

console.log(splitText('abcdefghijklmnop', 4));
/*  Abcd
    Efgh
    Ijkl
    Mnop
*/