JSFiddle - React, Tailwind, and code Playground

HTML

<canvas id="grayscaleCanvas" class="greyscaleCanvas" width="257px" height="257px"></canvas>

CSS

.greyscaleCanvas {
  border: solid 1px black;
}

JavaScript

function HeightMap(sideLength, maxHeight, roughness) {

  const n = Math.log(sideLength - 1) / Math.log(2);
  if (n < 0 || n % 1 != 0) {
    throw "Invalid side length in Diamond Square: Side Length has to be in range of `(2^n) + 1`.";
  }

  let gridArray = new Array(sideLength);
  for (var i = 0; i < gridArray.length; i++) {
  	gridArray[i] = new Float32Array(sideLength);
	}
  gridArray = _initGrid(gridArray, sideLength, maxHeight);
	gridArray = _seed(gridArray, sideLength, roughness);
  return gridArray;
}


function _initGrid(gridArray, sideLength, maxHeight) {

  gridArray[0][0] = RandomInt(0, maxHeight); // NW
  gridArray[0][sideLength-1] = RandomInt(0, maxHeight); // NE
  gridArray[sideLength-1][sideLength-1] = RandomInt(0, maxHeight); // SE
  gridArray[sideLength-1][0] = RandomInt(0, maxHeight); // SW

  return gridArray;
}


function _seed(gridArray, sideLength, roughness) {
  let step = sideLength - 1;
  let size = sideLength - 1;
  let currentRoughness = roughness;

	let run_num = 0
  while (step / 2 >= 1) {
    console.log(run_num)
		run_num = run_num + 1
    let numSquares = Math.pow(size, 2) / Math.pow(step, 2);
    let perRowSquares = Math.floor(Math.sqrt(numSquares));
    for (let i = 0; i < perRowSquares; i++) {
      for (let j = 0; j < perRowSquares; j++) {
        row = i*step
        col = j*step
        const squareCornerValues = _getSquareCornerValues(gridArray, row, col, step)
        gridArray = _diamondStep(squareCornerValues, row, col, step, gridArray, currentRoughness);
        gridArray = _squareStep(row, col, step, gridArray, sideLength, currentRoughness)
       // _squareStep(diamondMidPoints, gridArray, step, currentRoughness);
      }
    }

    currentRoughness /= 2.0;
    step /= 2;
  }
  return gridArray;
}


function _diamondStep(squareCornerValues, i, j, step, gridArray, currentRoughness) {
  gridArray[row+step/2][col+step/2] = (squareCornerValues[0] + squareCornerValues[1] + squareCornerValues[2] + squareCornerValues[3] ) /...