Simple Tween

tween.js

function tween(finalPos, prevPos, tweenRatio) {	
	if (finalPos === prevPos) {
		return prevPos;	
	} else {
		let dist = Math.abs(finalPos - prevPos);

		if (dist < 0.005) {
			// set to final position when getting close
			return prevPos;	
		} else {
			// tween
			let approach = dist * tweenRatio;

			if (prevPos < finalPos) 
				return prevPos + approach;
			else 
				return prevPos - approach;
		}
	}
};

module.exports = tween;

Using this in my threejs animation cycle like so:

        mesh.rotation.y = tween(state.origRotX + state.ratioX, mesh.rotation.y, 0.08);
        mesh.rotation.x = tween(state.origRotY + state.ratioY, mesh.rotation.x, 0.08);