Slider Moving Problem

I encounter a problem in dealing with slider bar moving in javascripts. The case is that I have created slider bar with ,I have four functionalities in slider bar functions
Please see the below image

(+)----------------|--------------(-)

1)when I click the (+) button the slider is moving (+) direction , when I click (-) button the slider is moving (-) direction .This process was working fine.


2)But when I click the mid bar slider right position the slider will go to (-) position and then when I click the mid bar slider left position , the slider will go to (+) position. This is concept. But when I click the mid bar both direction’s the slider is going (-) direction only. Code: