There is something simple about subtraction that has me stumped

[mangurian]

In the following javascript code snippet, I am getting the negative of the desired result.  In other words, the results in the alert are  selCenterX - 3000.  and selCenterY - 3000.

alert(selCenterX,selCenterY);
	var distancetoMidpointX = (3000. - selCenterX);
	var distancetoMidpointY = (3000. - selCenterY);
alert(distancetoMidpointX,distancetoMidpointY);

Something simple I am sure.  Please help.

[c.pfaffenbichler]

You seem to be using a comma in the alert where it does not quite belong. 

Try 

alert(distancetoMidpointX+","+distancetoMidpointY);

instead

[mangurian]

Here is the latest code.  Still the same problem - the result is the negative of the correct answer.

alert(selCenterX+","+selCenterY);
	var halfCanvas = 3000.
	var distancetoMidpointX = (halfCanvas - selCenterX); // assumes a 6k by 6k canvas
	var distancetoMidpointY = (halfCanvas - selCenterY);
alert(distancetoMidpointX+","+distancetoMidpointY);

[mangurian]

This is what the alerts return:

[jazz-y · Accepted solution]

I can't check right now, but maybe the problem is that you don't specify the units for halfCanvas (your selCenterX and selCenterX are both have type 'px'), those something like:

halfCanvas = new UnitValue( "3000 pixels" )

[mangurian]

Excellent.  I would never have found this on my own.  The solution was not simple to a novice.

Thank you again.

[c.pfaffenbichler]

I cannot reproduce the issue, please post the code properly.