Thanks, Loktar!

(function() {
    var requestAnimationFrame = window.requestAnimationFrame || window.mozRequestAnimationFrame ||
    window.webkitRequestAnimationFrame || window.msRequestAnimationFrame;
    window.requestAnimationFrame = requestAnimationFrame;
})();

var canvas = document.getElementById("canvas"),
    width = canvas.width,
    height = canvas.height,
    ctx = canvas.getContext("2d"),
    player = {
        x: 0,
        y: canvas.height - 25,
        velX: 0,
        velY: 0,
        speed: 3,
        width: 25,
        height: 25,
        friction: 0.9,
        gravity: 0.2,
        jumping: false
    },
    keys = [];
document.body.addEventListener("keydown", function(e) {
    keys[e.keyCode] = true;
});

document.body.addEventListener("keyup", function(e) {
    keys[e.keyCode] = false;
});
function rect(x, y, width, height, color) {
    ctx.fillStyle = color;
    ctx.fillRect(x, y, width, height);
    ctx.fillStyle = 'black';
}
function update() {
    if (keys[87]) {
        if (!player.jumping) {
            player.jumping = true;
            player.velY = -player.speed * 2;
        }
    }
    if (keys[68]) {
        if (player.velX < player.speed) {
            player.velX++;
        }
    }
    if (keys[65]) {
        if (player.velX > -player.speed) {
            player.velX--;
        }
    }
    if (player.x >= canvas.width - player.width) {
        player.x = canvas.width - player.width;
    } else if (player.x <= 0) {
        player.x = 0;
    }
    if (player.y >= canvas.height - player.height) {
        player.y = canvas.height - player.height;
        player.jumping = false;
    }
    player.x += player.velX;
    player.y += player.velY;
    player.velX *= player.friction;
    player.velY += player.gravity;
    ctx.clearRect(0, 0, canvas.width, canvas.height);
    rect(player.x, player.y, player.width, player.height, 'black');
    requestAnimationFrame(update);
}
requestAnimationFrame(update);