Now that I looked at it a little bit more, I understand that you want to highlight the searched term if it is found. In that case indexOf returns a number so this should be working with the end of word, which can be searchTerm.lenght and the call to select a range could be setSelectionRange(indexOfFirst, searchTerm.lenght). But anyway, you have to provide two numbers minimum, you can't get away with only one.
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Now that I looked at it a little bit more, I understand that you want to highlight the searched term if it is found. In that case
indexOfreturns a number so this should be working with the end of word, which can besearchTerm.lenghtand the call to select a range could besetSelectionRange(indexOfFirst, searchTerm.lenght). But anyway, you have to provide two numbers minimum, you can't get away with only one.