/**
 * Truncates `string` if it's longer than the given maximum string length.
 * The last characters of the truncated string are replaced with the omission
 * string which defaults to "...".
 *
 * @static
 * @memberOf _
 * @since 4.0.0
 * @category String
 * @param {string} [string=''] The string to truncate.
 * @param {Object} [options={}] The options object.
 * @param {number} [options.length=30] The maximum string length.
 * @param {string} [options.omission='...'] The string to indicate text is omitted.
 * @param {RegExp|string} [options.separator] The separator pattern to truncate to.
 * @returns {string} Returns the truncated string.
 * @example
 *
 * _.truncate('hi-diddly-ho there, neighborino');
 * // => 'hi-diddly-ho there, neighbo...'
 *
 * _.truncate('hi-diddly-ho there, neighborino', {
 *   'length': 24,
 *   'separator': ' '
 * });
 * // => 'hi-diddly-ho there,...'
 *
 * _.truncate('hi-diddly-ho there, neighborino', {
 *   'length': 24,
 *   'separator': /,? +/
 * });
 * // => 'hi-diddly-ho there...'
 *
 * _.truncate('hi-diddly-ho there, neighborino', {
 *   'omission': ' [...]'
 * });
 * // => 'hi-diddly-ho there, neig [...]'
 */
export default function truncate(str: string, separator: string | RegExp, length?: number, omission?: string): string;