longest substring with unique characters

Question

Given a string, please find the longest substring that has no repeated characters.

If there are multiple result, any one substring is fine.

longestUniqueSubstr('aaaaa')
// 'a'

longestUniqueSubstr('abcabc')
// 'abc', or 'bca', or 'cab'

longestUniqueSubstr('a12#2')
// 'a12#'

Follow-up

What is the time & space cost of your solution ? Could you do it better?

Code

/**
 * @param {string} str
 * @return {string}
 */
function longestUniqueSubstr(str) {
  // your code here
}

Source

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