[Solved] The way to increase variable

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You’re currently appending the number to the end of the string, this has nothing to do with arithmetic.

Just add the calculated result 

foo.setAttribute("item-position", bar+1);

You don’t have to turn it into a string, setAttribute will do that part.

Or if you want to increase the value in bar and show it, use the preincrement operator:

foo.setAttribute("item-position", ++bar);

solved The way to increase variable