Changing the position of an object on a map

I need a logic that, detecting if the value of x, y has changed, change the position of an object on the map.

I have a map that contains 50x50 tiles , and in these tiles there are objects that I can move using the code below:

var playerx = 10;
var playery = 15;

function drawMap() {
    for (i = 0; i < map.length; i++) {
        for (j = 0; j < map[i].length; j++) {
            var drawTile = map[i][j];
            var drawObj = objectMap[i][j];
            var xpos = (i-j) * tileH + mapX;
            var ypos = (i+j) * tileH/2 + mapY;

            ctx.drawImage(tileImg[drawTile], xpos, ypos);

            if (drawObj) {
                ctx.drawImage(objectImg[drawObj-1], xpos, ypos - (objectImg[drawObj-1].height));
            }
        }
    }
}

To add the object I use this property:

objectmap[playerx][playery] = 1;

And to remove I use the delete function:

delete objectmap[playerx][playery];

What happens is that the object is in positions 10 and 15 which are the values of the variables playerx and playery , but I have a function that takes the position of the mouse and clicking changes to the value of playerx and playery , it happens that as the value of the function changes, it adds objects to the map, without removing the last.

So the map has several objects, but I want the object to be just where the values of the variables point to.

Do you know how to do this with if or otherwise?