CODEWITHSUNDEEP
javascriptfor-looptype-conversion
Tadek
Tadek

Reputation: 71

var i = [0] in a for loop, then incremented i++ - why does it work?

In a book that I'm reading (JavaScript & JQuery - Interactive Front End Development by Jon Duckett) there's an interesting error or (at least I think so) which doesn't stop the code from working:

for (var i = [0]; i < options.length; i++) {         
   addEvent(options[i], 'click', radioChanged);       
}

This is a part of script that loops through all radio buttons in a form and attaches an event listener (it doesn't really matter what it does).

But...

  1. Why is i initialised as an array at all?

  2. Why does the incrementation work?

  3. Why does the whole loop work?

Of course if you replace var i = [0] with var i = 0 the code still works.

When you add some alerts to check the value of i in each iteration of the loop and the type of i, at the second iteration type of i changes from object (after all in the first iteration it is an array) to number. That's a kind of implicit type conversion I have never come across so far (and google don't help much). Can anyone explain what's going on under the hood?

for (var i = [0]; i < options.length; i++) {         
   addEvent(options[i], 'click', radioChanged);  
   alert(i); // --> 1   2   3 ...
   alert(type of i); // --> object   number   number ...
}

Upvotes: 7

Views: 330

Answers (1)

SLaks
SLaks

Reputation: 887797

The spec says (§ 11.3.1) that the ++ operator converts its operand to a number before incrementing:

  1. Let oldValue be ToNumber(GetValue(lhs)).

When called on an object, the GetValue internal operation will call toString(), which, for an array, will join its elements, returning '0'.

Upvotes: 8

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