conditionalRemoveFriendByValue.js

// helper function to return the error in a promise
async function updateWithErrorWrapper(params) {
  return new Promise(resolve => {
    DynamoDB.update(params, (err, data) => {
      resolve({ err, data });
    });
  });
}

async function conditionalRemoveFriendByValue(friendName) {
  const Key = { id: "1234" };
  
  // fetch the document
  let result = await DynamoDB.get({
    TableName,
    Key
  }).promise();
  
  // find the index
  let indexToRemove = result.Item.friends.indexOf(friendName);
  if (indexToRemove === -1) {
    // element not found
    return false;
  }
  
  // remove-by-index IFF the attribute contains the element we want to remove.
  const { err, data } = await updateWithErrorWrapper({
    TableName,
    Key,
    UpdateExpression: `REMOVE friends[${indexToRemove}]`,
    ConditionExpression: `friends[${indexToRemove}] = :valueToRemove`,
    ExpressionAttributeValues: {
      ":valueToRemove": friendName
    }
  });
  
  if (err) {
    if (err.code === "ConditionalCheckFailedException") {
      console.error("condition expression failed");
    } else {
      console.error("unhandled error: " + err);
    }
    return false;
  }
  return true;
}