Question

I know that there are many similiar questions already asked on stackoverflow, but still, I can not figure out how to do it. I want to rotate only ball texture. draw is called with timer:

var canvas;
var ctx;
var width;
var height;
var ready;
var textures;
var loadIndex;
var loadCount;
var keyCodes;
var mouseLoc;

var playerX;
var playerY;
var playerVelocity;

function init() {
    canvas = document.getElementById('game');
    ctx = canvas.getContext("2d");
    width = canvas.width;
    height = canvas.height;
    textures = [];
    loadingCount = 0;
    keyCodes = [];
    mouseLoc = {};

    playerX = 0;
    playerY = 0;
    playerVelocity = 6;

    textures['Background'] = loadTexture('./textures/Background.png');
    textures['Ball'] = loadTexture('./textures/Ball.png');

    setInterval(function(){ 
        if(loadingCount == 0) {
            update();
            draw();
        }
    }, 50);
}

function update(){
    if(keyCodes[37])
        playerX -= playerVelocity;
    if(keyCodes[38])
        playerY -= playerVelocity;
    if(keyCodes[39])
        playerX += playerVelocity;
    if(keyCodes[40])
        playerY += playerVelocity;
}

function draw() {
    //ctx.clearRect(0, 0, width, height);
    //ctx.beginPath();
    drawBackground();
    drawPlayer();
    //ctx.closePath();
    //ctx.fill();
}

function drawBackground(){
    ctx.drawImage(textures['Background'], 0, 0, width, height);
}

function drawPlayer(){
    ctx.save();
    ctx.rotate(0.17);
    ctx.drawImage(textures['Ball'], playerX, playerY, 100, 100);
    ctx.restore();
}

function loadTexture(src){
    var image = new Image();
    image.src = src;
    loadingCount++;
    image.onload = function(){
        loadingCount--;
    };
    return image;
}

document.onkeydown = function(evt){
    keyCodes[evt.keyCode] = true;
    evt.returnValue = false;
}
document.onkeyup = function(evt){
    keyCodes[evt.keyCode] = false;
}

document.onmousemove = function(evt){
    mouseLoc.x = evt.layerX;
    mouseLoc.y = evt.layerY;
}
document.onmousedown = function(evt){
    mouseLoc.down = true;
}
document.onmouseup = function(evt){
    mouseLoc.down = false;
}

init();

Answer 1

Assuming that you want to give the illusion of the ball continuing to rotate, you should increase the rotation angle for each frame drawn.

As written, your code will give the ball a fixed rotation of 0.17 radians on each frame.

var frame = 0;

function drawPlayer() {
    ctx.save();
    ctx.rotate(0.17 * frame);
    ...
    ctx.restore();
}

function draw() {
    ++frame;
    drawPlayer();
    ...
}