JavaScript已知起点与终点坐标获取单位向量

class utils{
    public static getVelocity(target:number[], position:number[]):number[] {
        // 由两点的位置计算速度的方向,速度大小不变
        let vel = 12;
        let b = position[0] - target[0];
        let a = position[1] - target[1];
        let speedY = Math.sqrt(vel/(1+Math.pow(b,2)/Math.pow(a,2)));
        let speedX = Math.sqrt(vel/(1+Math.pow(a,2)/Math.pow(b,2)));
        let velocity = [b>0?speedX:-speedX,a>0?speedY:-speedY];
        return velocity;
    }
}