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Question 54

What does [NaN].indexOf(NaN) return?

  1. -1Correct answer
  2. 0
  3. undefined
  4. true

Explanation

indexOf cannot match NaN when the search value is NaN, so it returns its no-match sentinel, -1. Treating it like includes ignores that the two methods use different equality behavior for this special numeric value.

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Question 54: What does [NaN].indexOf(NaN) return? · JavaScript Fundamentals · Questena