通过javascript

Question

我试图通过函数方法更改属性的值。我有它的工作但不完全是我需要它如何工作。现在基于用户输入，它将数字添加到速度值。但不是以数学的方式。相反，它只是将它添加到值的末尾。所以不是做12 + 2而是得到14.我得到122.如何让它一起添加值？

HTML

<form>      
    <input type="number" id="speed" min="-6" max="15" required>
    <input type="button" id="gear_submit" value="Submit" onclick="createBike();">
</form>
<p id="show_speed"></p>

的Javascript

function createBike(){
    function bike(model, speed){
        this.model = model;
        this.speed = speed;

    // this will change speed based on user input
        this.changeSpeed = function (changeSpeed) {
            var new_speed = document.getElementById("speed").value;
            if (new_speed > 0 ){
                bikeArray[0].speed = speed + new_speed;
            }
            else if (new_speed < 0 ){
                bikeArray[0].speed - speed - new_speed;
            }
        }
    }

    var bike1 = new bike("Ghost Ryder", "12");
    bikeArray[0] = bike1;

    bike1.changeSpeed();

    document.getElementById("show_speed").innerHTML = bikeArray[0].model + " " + bikeArray[0].speed;
}