You could use this...

Math.abs(x)

Math​.abs() | MDN

Use Math.abs() :

var x = -25;
alert(Math.abs(x)); //it will alert 25

Here are some test cases from the documentation:

Math.abs('-10');     // 10
Math.abs(-20);       // 20
Math.abs(null);     // 0
Math.abs("string"); // NaN
Math.abs();         // NaN

As mentioned in other comments, you won't be able to keep the precision. toFixed() is definitely not a solution and this problem has valid case scenarios.

We had to do something similar where teachers would input a grade (90.05) and were expected to keep the same number of decimals for all their students inside of a single gradebook. Teachers grade differently depending of the situation:

Teacher A | Teacher B | Teacher C
   90          90.5       4.95
   80          88.5       3.80
   75          80.0       5.00

One way to solve this, especially if the request comes as a string (from an input, http request, etc.), is to maintain the number as a string and return a string value.

Note that this is a very crude implementation (our implementation is more fail-safe), but the idea is simple:

function absString(n) {
  numberString = n.toString();
  if (numberString[0] === '-') {
    return numberString.substring(1);
  }
  else {
    return numberString;
  }
}

console.log(absString('-1'));
console.log(absString('-2.0'));
console.log(absString('2.0'));
console.log(absString('-0.57'));

There are various ways:

var m, n = -25;
m = -1 * n;
m = -n;
m = Math.abs(n);

Math.abs(num) => Always positive
-Math.abs(num) => Always negative

You do realize, however, that for your code:

if($this.find('.pdxslide-activeSlide').index() < slideNum-1) {
   slideNum = -slideNum;
}

console.log(slideNum)

If the index found is 3 and slideNum is 3,
then 3 < 3-1 => false
so slideNum remains positive??

It looks more like a logic error to me.

You can do a simple math at this context, no need of Math.abs,

x_value = x_value * -1;

Or you can negate the value like,

x_value = -(x_value);

While negating, there is a chance to get -0, But we don't need to worry about it, since -0 == 0. Abstract equality comparison algorithm is telling so in Step 1 - c - vi.