substr.js
function substr(string, start, length) {
    if (!isString(string)) {
        return string;
    }

    return string.substr(start, length);
}
function substr(string, start, length) {
    if (!isString(string)) {
        return string;
    }

    return string.substr(start, length);
}
Extracts parts of a string, beginning at the character at the specified position,
and returns the specified number of characters.
The substr() does not change the original string.

@customNeeds Validate string type for preventing SyntaxError

@since 1.0.0
@category String

@param {String} string -  The string to extract
@param {Number} start -  The position where to start the extraction. First character is at index 0
@param {Number?} length -  Optional. The number of characters to extract. If omitted, it extracts the rest of the string

@returns {String} Returns extract part of a string

@example

famulus.substr('Hello World!', 0, 5)
// => 'Hello'

famulus.substr('Hello World!', 6)
// => 'World!'