Simple example to get started with a browser based game
Instead of using ball_pos as an array with two elements to contain the x and y coordinate values of the ball,
it would be more natural to use a an object like this:

ball = {
    x: canvas.width() / 2,
    y: canvas.height() / 2
}


Instead of writing keys[37] and explaining with a comment // left what the number means,
it would be better to put that in a well-named variables, for example:

var KEY_LEFT = 37;


Then you can rewrite the conditions like this:

if (keys[KEY_LEFT]) {
    ball.x -= vel; 
}
if (keys[KEY_UP]) {
    ball.y -= vel;
}


Putting it together:



var canvas, context, ball, vel, keys = {};

var BALL_RADIUS = 20;
var KEY_LEFT = 37;
var KEY_UP = 38;
var KEY_RIGHT = 39;
var KEY_DOWN = 40;

function init() {
    'use strict';
    canvas = document.getElementById('c');
    context = canvas.getContext('2d');
    ball = { x: canvas.width / 2, y: canvas.height / 2 };
    vel = 4;
}

function draw() {
    'use strict';

    // draw background
    context.fillStyle = 'black';
    context.fillRect(0, 0, canvas.width, canvas.height);

    // draw ball
    context.strokeStyle = 'red';
    context.lineWidth = 2;
    context.fillStyle = 'white';
    context.beginPath();
    context.arc(ball.x, ball.y, BALL_RADIUS, 0, Math.PI * 2, true);
    context.closePath();
    context.fill();
    context.stroke();
}

function keydown(evt) {
    'use strict';
    // mozilla based browsers return which and others keyCode
    var keyCode = evt.which || evt.keyCode;
    keys[keyCode] = true;
    // alert(keyCode)
    // 33 up right
    // 34 down right
    // 35 down right
    // 36 up left
    if (keys[KEY_LEFT]) {
        ball.x -= vel; 
    }
    if (keys[KEY_UP]) {
        ball.y -= vel;
    }
    if (keys[KEY_RIGHT]) {
        ball.x += vel;
    }
    if (keys[KEY_DOWN]) {
        ball.y += vel;
    }
}

function keyup(evt) {
    'use strict';
    var keyCode = evt.which || evt.keyCode;
    delete keys[keyCode];
}

function animate() {
    'use strict';
    window.requestAnimationFrame(animate);
    draw();
}

// register event handlers
window.onkeydown = keydown;
window.onkeyup = keyup;

init();
animate();
&lt;!DOCTYPE HTML&gt;
&lt;html&gt;
&lt;head&gt;
  &lt;meta charset="utf-8"&gt;
&lt;/head&gt;
&lt;body&gt;
  &lt;canvas id="c" width="600" height="400"&gt;&lt;/canvas&gt;
&lt;/body&gt;
&lt;/html&gt;