javascript - js keybourd 函数 - 事件未定义

Question

这段代码有什么问题？为什么 console.log(evt) 返回“undefined”？

这是我的代码:

var documentKeydown = (evt)=>{
        console.log(evt);
        if (counter == 0) {
            XWhenDown = currentX;
            YWhenDown = currentY;
            document.onkeyup = documentKeyup();
            switch (evt.keyCode) {
                case 37 : currentX -= 10; break;
                case 38 : currentY -= 10; break;
                case 39 : currentX += 10; break;
                case 40 : currentY += 10; break;
            }
            animationTick = Ticker.add(animation);
        }
    }

    document.onkeydown = documentKeydown();

Answer 1

你应该只在事件发生时调用你的函数:

var documentKeydown = ()=>{
    console.log(event);
    if (counter == 0) {
        XWhenDown = currentX;
        YWhenDown = currentY;
        document.onkeyup = documentKeyup();
        switch (event.keyCode) {
            case 37 : currentX -= 10; break;
            case 38 : currentY -= 10; break;
            case 39 : currentX += 10; break;
            case 40 : currentY += 10; break;
        }
        animationTick = Ticker.add(animation);
    }
 }

document.addEventListener('keydown', documentKeydown) ;