Javascript under-the-hood toString conversion

Question

I have the following function:

function addChange(result, bill) {
  for (var i=0;i<result.length;i++) {
    if (result[i][0] === bill[0]) {
      result[i][1] = parseFloat(result[i][1]).toPrecision(2) + parseFloat(bill[1]).toPrecision(2);
      return result;
    }
  }
  result.push(bill);
  return result;
}

Which is called as following, for example:

result = [["QUARTER", 0.25],["DIME", 0.10]];
bill = ["QUARTER", 0.25];
addChange(result, bill);

The output to the above is:

[["QUARTER", "0.250.25"], ["DIME", 0.10]] 

Why does javascript convert those floats into strings before making the addition? Or is there something else going on? How can I move past it and achieve what I want, i.e., for the above example:

[["QUARTER", 0.50], ["DIME", 0.10]]

Answer 1

The Problem is:

floatNumber.toPrecision(2)

This creates a string from your number. The toPrecision-function is used to create a representation of your number with a fixed number of digits.

Answer 2

From the mdn documentation https://developer.mozilla.org/en/docs/Web/JavaScript/Reference/Global_objects/Number/toPrecision

Returns

A string representing a Number object in fixed-point or exponential notation rounded to precision significant digits. See the discussion of rounding in the description of the Number.prototype.toFixed() method, which also applies to toPrecision().