how to I find the Right most value in a linear matrix?

this is JavaScript in HTML using 2d context

If I have a linear matrix, how do I find the right most value? Currently I'm guessing my way around Wave function collapse and I need to add boundaries when they are checking their neighbors. but what I want to know is how to check the position to the right exist?

//3 columns and rows
var DIM = 3;

//30 pixels wide
var size = 30;

//making an empty array for matrix
var matrix = [];

//filling in the matrix with identifier for drawing
for(let i=0; i<DIM*DIM; i++)
{
   matrix[i] = 0;
};

//checking the neighbors
//gonna add more rules later
//i have more indetifier other than 1 and 0
//id like to add rules like a 1 can only have 2 or 5 above it
for(let i=0; i<DIM*DIM; i++)
{
   let cell = grid[i];
   //if the neighbor is null ignore
   let aboveCell = null;
   let belowCell = null;
   let rightCell = null;
   let leftCell = null;

   //Boundaries
   if(i <= DIM)
      aboveCell = matrix[i-DIM];

   if(i <= grid.length-DIM)
      belowCell = matrix[i+DIM];

   if(i % DIM != 0)
      leftCell = matrix[i-1];

   //HERE IS WHERE I WANT TO KNOW HOW TO CHECK THAT IT DOESNT HAVE A RIGHT NEIGHBOR

}

//this draws the matrix on the canvas
function draw()
{
  //running through the matrix to draw the tiles in the right spot
  for(let x=0; x<DIM; x++)
  {
     for(let y=0; y<DIM; y++)
     {
         if(matrix[x+(y*DIM)] == 0)
         {
            ctx.fillStyle= "black";
            ctx.fillRect(x*size, y*size, size, size);
         }
         if(matrix[x+(y*DIM)] == 1)
         {
            ctx.fillStyle= "white";
            ctx.fillRect(x*size, y*size, size, size);
         }
      };
   };


}

I've tried a few equations(made by me) but clearly the haven't worked

You can check if the remainder of the global index per the width of the matrix is equal to the width minus one:

const WIDTH = 30;
// By default an ImageData is such a flat "matrix".
const img = new ImageData(WIDTH, 50);
// But its initial Uint8 buffer gives us access to
// the 4 channels of each RGBA pixel.
// Using an Uint32 view each index represents a full pixel,
// matching OP's situation.
const data = new Uint32Array(img.data.buffer);

for (let i = 0; i<data.length; i++) {
  data[i] = i % WIDTH === WIDTH - 1
    ? 0xFFFF00FF // Right-most: purple
    : Math.random() * 0xFFFFFF + 0xFF000000; // random
}

document.querySelector("canvas").getContext("2d").putImageData(img, 0, 0);

canvas {
  width: 300px;
  height: 500px;
  image-rendering: pixelated;
}

<canvas width="30" height="50"></canvas>