Modify object properties in javascript if condition applies

Game

So i'm making a game, if i catch the ball i should go to the next level and the plank under the cat should go lower.

this is my plank object.

 var Plank = function (c, top, fill) {
                this.left = 20;
                this.top = top;
                this.width = c.width - 52;
                this.height = 6;
                this.fill = fill;
                this.draw = function () {
                    var rectangle = new fabric.Rect({
                        left: this.left,
                        top: this.top,
                        fill: this.fill,
                        width: this.width,
                        height: this.height
                    });
                    rectangle.selectable = false;
                    c.add(rectangle);
                };
            };

I make a new object instance.

var plank = new Plank(canvas, canvas.height / 4 + 20, '#FFFF00');
        plank.draw();

And here's how i define what's gonna happen if the ball gets caught.

function checkBalls() {
        for (var i = 0; i < balls.length; i++) {
            console.log(balls);
            var ball = balls[i];



            var x = ball.pokeBall.left;
            var y = ball.pokeBall.top;


            if (ball.hasFallen) {
                //remove from array
                ball.pokeBall.ball = null;
                balls.splice(i, 1);
            }
            if (y > 400 && ball.hasFallen == false) {
                var pikaPosition = somePika.pika.left;
                var pikaPadding = 50;

                var plankpos = plank.top;

                if ((pikaPosition - pikaPadding) < x && x < (pikaPosition + pikaPadding)) {
                    ball.hasFallen = true;
                    score++;
                    plankpos =+ 30;
                    elScore.textContent = score;

                        if (score == 2) {
                            alert("UP TO LEVEL 2! CATCH 10 MORE POKEBALLS.");
                            planktop += 10;


                        }

                        else if (score == 4) {
                            alert("LEVEL 3! NICE!");
                              planktop += 10;
                        }

                        else if (score == 6) {
                            alert("YOU WIN, CONGRATULATIONS!");
                            document.location.reload();
                        }
                }
            }
        }
    }

But for some reason i can't get the plank to move when i call its property.