. help with javarscrip pronlem

  • I downloaded a couple of Javascript tutorials about 'Baasic physics', and i'm stuck on page 1.

    Note. I have done a bit more research and think i understand most of it, but i can't work out what the symbols marked in red mean?

    // Version 3 - Basic collision (viewbox), perfectly elastic (no energy loss)

    var ball = {

    x: 300,

    y: 200,

    r: 15,

    vx: 0,

    vy: 0

    }

    // Update ball (with Physics! =)

    // 1 - apply velocity to position (vx -> x)

    ball.x += ball.vx;

    ball.y += ball.vy;

    // 2 - apply drag/friction to velocity

    ball.vx *= .99;

    ball.vy *= .99;

    // 3 - apply gravity to velocity

    ball.vy += .25;

    // 4 - check for collisions

    if (ball.y + ball.r > canvas.height) {

    ball.y = canvas.height - ball.r;

    ball.vy = -Math.abs(ball.vy);

    }

    // Draw ball

    ctx.save();

    ctx.translate(ball.x, ball.y);

    ctx.fillStyle = "#fff";

    ctx.beginPath();

    ctx.arc(0, 0, ball.r, 0, Math.PI * 2, true);

    ctx.closePath();

    ctx.fill();

    ctx.restore();

    }, 1000 / 77);

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