The `echo` part seemed interesting but it made me think, debuggers need this as a built in feature. If I have

    array
      .slice(0, 10)
      .filter(s => s[0].toLowerCase() < 'm') // a<->l
      .map(s => s.toUpperCase());

The out of bounds handling didn't seem all that good to me. Sure you can filter out undefined. You could also just make a function that returns an empty array if out of bounds, or array of 1 element if not.

     // JS
     function getElemFromGrid(grid, x, y) {
       return (x < 0 || x >= grid.width ||
               y < 0 || y >= grid.height)
         ? []
         : [grid.elems[y][x]]
     }
     
     ...
     
     neighbors = [
       ...getElemFromGrid(grid, x + 1, y + 0),
       ...getElemFromGrid(grid, x + 1, y + 1),
       ...getElemFromGrid(grid, x + 0, y + 1),
       ...getElemFromGrid(grid, x - 1, y + 1),
       ...getElemFromGrid(grid, x - 1, y + 0),
       ...getElemFromGrid(grid, x - 1, y - 1),
       ...getElemFromGrid(grid, x + 0, y - 1),
       ...getElemFromGrid(grid, x + 1, y - 1),
     ]