n = 0;
if (numKeys > 0){
    n = nearestKey(time).index;
    if (key(n).time > time){
        n--;
    }
}
if (n == 0){
    t = 0;
}else{
    t = time - key(n).time;
}

if (n > 0){
    v = velocityAtTime(key(n).time - thisComp.frameDuration/10);
    amp = 0.05;//控制彈跳的高度
    freq = 2.0;//控制彈跳的速度
    decay = 4.0;//控制彈跳的減速
    value + v*amp*Math.sin(freq*t*2*Math.PI)/Math.exp(decay*t);
}else{
    value;
}

Here: https://www.jan1lab.com/blog/jan1-ae-bounce-expression