ES6 training

substr.js

const substr = (str, start = 0, length = str.length) => {
    if (start === 0 && length === str.length) {
        return str;
    }

    if (start > str.length || length <= 0) {
        return '';
    }
    
    const startIndex = (start < 0) ? Math.max(0, str.length - Math.abs(start)) : start;
    const stopIndex = Math.min(str.length, startIndex + length);

    let result = '';

    for (let i = startIndex; i < stopIndex; i++) {
        result += str[i];
    }

    return result;
};