My goal is to code a somewhat OK chess engine, in the following position it's a mate in 2 which the engine should easily find with its depth of 4-5.

The first move the AI makes is Ra2 to trap the white king, the white king goes to f1 and instead of mating the AI moves the Rook to c2.

  var initial_depth = depth;
  var bestMove = null;
  var nodes = 0;
  var ret = await minimax(position, depth, alpha, beta, maximizingPlayer);
  console.log("nodes visited: " + nodes);
  return ret;
 
  async function minimax(position, depth, alpha, beta, maximizingPlayer) {
    nodes++;
    if (maximizingPlayer) {
      var validMoves = await getValidMoves(position, ArrtoFEN(position) + " w");
    } else {
      var validMoves = await getValidMoves(position, ArrtoFEN(position) + " b");
    }
    if (validMoves.length < 1 || depth == 0) {
      var eval = await getEval(position);
      return [eval, null];
    }
 
    if (maximizingPlayer) {
      var maxEval = Number.NEGATIVE_INFINITY;
      for (var i = 0; i < validMoves.length; i++) {
        var move = validMoves[i];
        
        var testbrd = makeMove(move, position)  //not the actual code. shortend for Readability
 
        
        var eval = await minimax(testbrd, depth - 1, alpha, beta, false);
        if (eval[0] > maxEval) {
          maxEval = eval[0];
          if (initial_depth == depth) {
            bestMove = move;
            console.log("current bestmove: " + bestMove);
          }
        }
        alpha = Math.max(alpha, eval[0]);
        if (beta <= alpha) {
          break;
        }
      }
      return [maxEval, bestMove];
    } else {
      var minEval = Number.POSITIVE_INFINITY;
 
      for (var i = 0; i < validMoves.length; i++) {
        var move = validMoves[i]; 
          
        var testbrd = makeMove(move, position)//not the actual code. shortend for Readability
 
        var eval = await minimax(testbrd, depth - 1, alpha, beta, true);
        if (eval[0] < minEval) {
          minEval = eval[0];
          if (initial_depth == depth) {
            bestMove = move;
            console.log("current bestmove: " + bestMove);
          }
        }
        beta = Math.min(beta, eval[0]);
        if (beta <= alpha) {
          break;
        }
      }
 
      return [minEval, bestMove];
    }
  }
}