function isIndex(value, length) {
    value = +value;
    return value > -1 && value % 1 == 0 && (length == null || value < length);
  }
Checks if `value` is valid array-like index.

@private
@param {*} value The value to check.
@param {number} [length] The upper bound of a valid index.
@returns {boolean} Returns `true` if `value` is a valid index, else `false`.