Question

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 is not a function

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?

Here is my code:

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);
}

Answer 1

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.

Instead, wrap your call inside an anonymous function:

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