Javascript - Don't understand data type of a variable

I have the following:

this.replaceBand1 = "1";
...
var replaceEndValue = Integer.valueOf(this.replaceBand1);
replaceBeginValue = replaceEndValue + 1;
//what is going on?
var type1 = typeOf replaceEndValue;
var type2 = typeOf replaceBeginValue;
log.debug("replaceEndValue: " + replaceEndValue + " " + type1);
log.debug("replaceBeginValue: " + replaceBeginValue + " " + type2);
...
setValue(column1, replaceEndValue);
setValue(column2, replaceBeginValue);

My expected outcome: replaceBeginValue will equal 2, and I can pass that into the function setValue that requires an Integer.

Actual outcome: replaceEndValue works, replaceBeginValue does not work.

The first debug shows - replaceEndValue: 1 object The second debug shows - replaceBeginValue 2 string

I have no idea why replaceBeginValue is a string. Can anybody help?