I'm trying to use threeJS to control a camera in my scene. I currently have the camera set up to orbit in a circle around my object using the left and right keys on my keyboard. But does anyone know how I would zoom? Here's my current code:

camera = new THREE.PerspectiveCamera(45, window.innerWidth / window.innerHeight, 1, 10000 );

camera.position.set(0,20,35);

var rotSpeed = .02;



function checkRotation(){



    var x = camera.position.x,

        y = camera.position.y,

        z = camera.position.z;



    if (keyboard.pressed(left)){ //MH - find a way to do this in a switch statement 

        camera.position.x = x * Math.cos(rotSpeed) + z * Math.sin(rotSpeed);

        camera.position.z = z * Math.cos(rotSpeed) - x * Math.sin(rotSpeed);

    } else if (keyboard.pressed(right)){

        camera.position.x = x * Math.cos(rotSpeed) - z * Math.sin(rotSpeed);

        camera.position.z = z * Math.cos(rotSpeed) + x * Math.sin(rotSpeed);

    } else if(keyboard.pressed(up)){

        //zoom in

    } else if (keyboard.pressed(down)){

        //zoom out

    }



    camera.lookAt(scene.position);



}