Does indexOf != -1 return a Boolean?

Question

I have just started learning Javascript and I have a specific question about a specific piece of code below. It is part of the lycanthrope's log in chapter 4 of Eloquent Javascript. Because of the specificity of my question I haven't included all the other code associated with this problem for I believe it isn't necessary to answer my question.

Please do let me know if this is considered 'bad practice' and I will make sure to ammend this and/or future posts to show more background.

In the code below the second line shows a return. So far I have learned that indexOf returns a positive number or zero if and only if it finds an occurence of whatever is passed in it. If no occurence is found it returns -1.

In this case it is followed by != -1, which I understand to mean "not equal to minus 1". That is clear to me.

What I do not completely understand is what the actual return in line 2 ends up being. Does it return a Boolean value of either true or false? Or does it return he actual index where the 'event' is found?

Further on, in the first if-statement, we see the hasEvent variable again. I read this statement as "If hasEvent(event, entry) is true then add 1 to the index.

Am I 'reading' this right and is the return in the second line indeed a Boolean?

function hasEvent (event, entry) {
  return entry.events.indexOf(event) != -1;
}

function tableFor (event, journal) {
 var table = [0, 0, 0, 0];
 for (var i=0; i < journal.length; i++) {
  var entry = journal[i] , index = 0;
  if (hasEvent(event, entry)) index += 1;
  if (entry.squirrel) index += 2;
  table [index] += 1;
}
 return table;
}

Thank you for your help and please tell me if I should have stated this question differently! I am trying to make sure I understand things before I move on!

Answer 1

The != operator always has a boolean result.

A return statement followed by an expression returns the value of the expression, so the returned value of that function will be either true or false.