TypeError：canvas.getContext不是一个函数 [英] TypeError: canvas.getContext is not a function

问题描述

我正在尝试使用canvas元素为跟随Javascript中光标的球设置动画。我将对我的canvas对象的引用传递给名为followMouse的函数，但是当我尝试获取其上下文时，firebug给了我这个错误：

I am trying to animate a ball that follows the cursor in Javascript using the canvas element. I pass a reference to my canvas object to the function named followMouse, but when I try to get its context, firebug gives me this error:

TypeError：canvas.getContext不是函数

TypeError: canvas.getContext is not a function

当我将画布引用登录到控制台时，它会显示canvas元素。我不知道为什么然后无法在函数中获取上下文。有人知道发生了什么吗？

When I log the canvas reference to the console, it displays the canvas element. I don't know why then I cannot get the context in the function. Anyone know what is going on?

这是我的代码：

function drawCircle(x, y, canvas) {
    var context = canvas.getContext('2d');
    context.beginPath();
    context.arc(x, y, 40, 0, 2 * Math.PI);
    context.fill(); 
}

function followMouse(canvas, mousePos) {
    var context = canvas.getContext("2d");
    console.log(canvas);
    context.clearRect(0, 0, 700, 700);
    var xPos = xPos + (mousePos.x - xPos) / 100;
    var yPos = yPos + (mousePos.y - yPos) / 100;
    drawCircle(xPos, yPos, canvas);
    window.requestAnimFrame(followMouse, canvas);
}

window.onload = function() {
    var canvas = document.getElementById("main");

    var mousePos = {
        x: 0,
        y: 0
    };

    canvas.addEventListener('mousemove', function(evt) {
           var pos = getMousePos(canvas, evt);
           mousePos.x = pos.x;
           mousePos.y = pos.y;
    }, false);

    followMouse(canvas, mousePos);
}

推荐答案

您的行 window.requestAnimFrame（followMouse，canvas）; 调用 followMouse（timestamp），其中timestamp是传递的 number 到 requestAnimationFrame 。这会导致调用 timestamp.getContext ，这显然是无效的。

Your line window.requestAnimFrame(followMouse, canvas); calls followMouse(timestamp), where timestamp is a number passed to the callback when it is fired by requestAnimationFrame. This results in a call to timestamp.getContext, which is obviously invalid.

相反，请将您的调用包装在匿名函数：

Instead, wrap your call inside an anonymous function:

window.requestAnimFrame(function() { followMouse(canvas, mousePos) }, canvas);

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