chess.spec.js

function isValidMove(turn, piece, startSquare, endSquare) {
  const startFile = startSquare[0];
  const endFile = endSquare[0];
  const startRank = startSquare[1];
  const endRank = endSquare[1];

  if (endRank - startRank > 1) {
    return false;
  }

  return startFile === endFile;
}

describe('Correct pawn moves for whites', function () {
  it('should be valid guess when same file and one rank ahead', function () {
    expect(isValidMove('WHITES', 'PAWN', 'e2', 'e3')).toEqual(true);
  });
  it('should be an error when same file but three rank ahead', function () {
    expect(isValidMove('WHITES', 'PAWN', 'e2', 'e5')).toEqual(false);
  });
  it('should be an error to be on a different file', function () {
    expect(isValidMove('WHITES', 'PAWN', 'e2', 'd2')).toEqual(false);
  });
  it('should be an error to be on a different file', function () {
    expect(isValidMove('WHITES', 'PAWN', 'e2', 'c2')).toEqual(false);
  });
});