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JavaScript

const string = 'stackoverflow';

function spliter(str, nr) {
    const parts = [];
    for (let i = 0, length = str.length; i < length; i += nr) {
        parts.push(str.substring(i, i + nr));
    }
    return parts;
}

console.log(spliter(string, 3)); // ["sta", "cko", "ver", "flo", "w"]
console.log(spliter(string, 2)); // ["st", "ac", "ko", "ve", "rf", "lo", "w"]
console.log(spliter(string, 5)); // ["stack", "overf", "low"]