Randomly generate += or -= javascript

Hi i am trying to assign either += or -= to an object position. Firstly I have random math choosing a number:

var min = 9;
 var max = 11;
 var pos = Math.floor(Math.random() * (max - min + 1)) + min;

I am trying to add the out come of pos to this._target.position.z and this._target.position.x but I want it to be randomly either added or subtracted

so the random out come should be for the Z position should be either:

this._target.position.z += pos;

or

this._target.position.z -= pos;

and then for the X position should be either:

this._target.position.x += pos;

or

this._target.position.x -= pos;

thanks in advance.