This is a common error. i is a global variable that you're incrementing in the for loop. When the loop finishes, i will be equal to 4. Then in your click handler function, you're trying to display arr[i].innerHTML, which is now arr[4].innerHTML. arr[4] obviously doesn't exist, so you get an error.
For an easy fix, change alert(arr[i].innerHTML) to alert(this.innerHTML). this, in the context of the click handler function, will refer to the | element.
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