find the first duplicate character in a string

Question

Given a string which might have duplicate letters, write a function to find the first duplicate.

firstDuplicate('abca')
// 'a'

firstDuplicate('abcdefe')
// 'e'

firstDuplicate('abcdef')
// null

What is the time & space cost of your approach ? Can you do better?

Code

/**
 * @param {string} str
 * @return {string | null}
 */
function firstDuplicate(str) {
  // your code here
}

Source

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