code: function checkForAtomicOperators(update) {
    const keys = Object.keys(update);
  
    // same errors as the server would give for update doc lacking atomic operators
    if (keys.length === 0) {
      return toError('The update operation document must contain at least one atomic operator.');
    }
  
    if (keys[0][0] !== '$') {
      return toError('the update operation document must contain atomic operators.');
    }
  }
docs: Check the update operation to ensure it has atomic operators.