画布javascript中的键检测

Question

我尝试绘制一个简单的矩形并通过键盘移动它。但问题是我认为在我的代码中添加了我需要的一切......好吧，我想在键盘上使用箭头。但在我试图获得警报之前......但它不起作用......请帮助我......任何帮助将不胜感激。

var canvas = document.getElementById("screen");
        context = canvas.getContext("2d");

function Player() {
        this.x=0, this.y = 0, this.w = 50, this.h = 50;
        this.render = function (){
            context.fillStyle = "orange";
            context.fillRect(this.x, this.y, this.w, this.h);
        }
      }                
      var player = new Player();

player.x=100;
            player.y= 460;
            setInterval( function() {
                context.fillStyle="black";
                context.fillRect(0,0,canvas.width, canvas.height);
                /*context.fillStyle = "white";
                context.fillRect(100, 460, 30 , 30);*/

                player.render();

                //move all aliens & draw all aliens
                for(var i = 0; i < 9; i++) {
                    aliens[i].move(),
                    aliens[i].draw(context);
                }
            }, 20);
            document.addEventListener('keydown', function(event)){
                var key_press = String.fromCharCode(event.keyCode);
                alert(event.keyCode + " | " + key_press);

            });
        }

Answer 1

document.addEventListener('keydown', function(event)){
----------------------------------------------------^

你确实在上面加了额外的括号，删除它，实际的代码是

document.addEventListener('keydown', function(event){