如果是边界的陈述

Question

我为我的汽车添加了一些if语句，但没有超出画布的界限，但它仍然过去，所以你能为我调试一下还是只告诉我有什么问题？提前谢谢。

JS： 代码无法正常工作

    function update() {
        window.requestAnimationFrame(update);
        x += speed;
        y += speed;

    if (x >= $('#gameCanvas').width() - x/2) {
        speed = 0;
      } else if (x <= x/2) {
        speed = 0;
      }
      x += speed;

      if (y >= $('#gameCanvas').height() - y/2) {
        speed = 0;
      } else if (y <= y/2) {
        speed = 0;
      }
      y += speed;

       render();
    }


Total js code:


var canvas = document.getElementById('gameCanvas');
var context = canvas.getContext('2d');
var car = new Image();
car.src = "http://thumbs.dreamstime.com/x/compact-yellow-car-top-view-10334412.jpg";

var x = 220;
var y = 200;
var speed = 5;
var angle = 0;
var mod = 0;

window.addEventListener("keydown", keypress_handler, false);
window.addEventListener("keyup", keyup_handler, false);

var moveInterval = setInterval(function () {
    draw();
}, 30);


function draw() {
    context.clearRect(0, 0, canvas.width, canvas.height);

    x += (speed * mod) * Math.cos(Math.PI / 180 * angle);
    y += (speed * mod) * Math.sin(Math.PI / 180 * angle);

    context.save();
    context.translate(x, y);
    context.rotate(Math.PI / 180 * angle);
    context.drawImage(car, -(car.width / 2), -(car.height / 2));
    context.restore();
}

function update() {
    window.requestAnimationFrame(update);
    x += speed;
    y += speed;

if (x >= $('#gameCanvas').width() - x/2) {
    speed = 0;
  } else if (x <= x/2) {
    speed = 0;
  }
  x += speed;

  if (y >= $('#gameCanvas').height() - y/2) {
    speed = 0;
  } else if (y <= y/2) {
    speed = 0;
  }
  y += speed;

   render();
}

function keyup_handler(event) {
    console.log('a');
    if (event.keyCode == 38 || event.keyCode == 40) {

        mod = 0;
    }
}

function keypress_handler(event) {
    console.log(event.keyCode);
    if (event.keyCode == 38) {
        mod = 1;
    }
    if (event.keyCode == 40) {
        mod = -1;
    }
    if (event.keyCode == 37) {
        angle -= 5;
    }
    if (event.keyCode == 39) {
        angle += 5;
    }
}