Using offsets. More dry. https://repl.it/@ambergkim/nearestneighbors

getNeighbors2.js

class Grid {
  // determine number of rows and columns.
  constructor(rows, columns) {
    this.rows = rows;
    this.columns = columns;
  }
  
  // get the neighbors helper
  getNeighbors(row, column) {

    // check if source is valid.
    let isValid = this.isValidCoord(row, column);

    // if it's not valid.
    if (!isValid) {
      return undefined;
    }
    
    // offsets relative to the coordinate
    let offsets = [
      [-1, -1], [-1, 0] , [-1, 1],
      [0,  -1],           [ 0, 1],
      [1,  -1], [1, 0] ,  [ 1, 1]
    ];
       

    let neighbors = offsets
    // Add the offsets to the coordinate to get the neighbors.
    .map( ([dy, dx]) => [row + dy, column + dx])
    // filter out invalid neighbors
    .filter( ([y,x]) => this.isValidCoord(y,x));
        
    return neighbors;
  }

  // verify coordinate validity
  isValidCoord(y, x) {
    if (y >= 0 && y < this.rows && x >= 0 && x < this.columns) {
      return true;
    }
    return false;
  }
}

let grid1 = new Grid(4, 4);
console.log('grid1 size rows', grid1.rows, 'columns', grid1.columns);

let neighbors = grid1.getNeighbors(3, 2);

console.log('neighbors ', neighbors);