update attributes in a dynamodb record

I am trying to update my existing record in my dynamodb table. Like below I have an Item in my table

let params = {
   TableName: proces.env.dynamoDbTable,
   Item: {
      productId: "id",
      att1: val1,
      att2: val2
      }
 }

I want to perform an update. I am using the aws dynamodb sdk's update method and passing it params like below

let aws = require('aws-sdk');
 let dbb = new aws.DynamoDb.DocumentClient();
 let params = {
  TableName: process.env.tableName,
  Key: {productID}
  ExpressionAttributeNames: { "#updatedAt" : "updatedAt" }
  ExpressionAttributeValues: {":u":moment().unix(), ":val1" : a, ":val2": b}
  UpdateExpression: "SET att1 = :val1, att2: val2, #updatedAt: :u"
 }
// a, b are passed as argument to function and are optional
dbb.update(params).promise()

When an argument goes missing the dynamo raises ExpressionAttributeValue missing exception and I know it is straight. Is there a way I can update my Item with the attributes provided

Unfortunately dynamodb does not make this easy. But you can use some fancy js to create the params object dynamically:

  // for some object `attrs` we find which keys we need to update
  const keys = Object.keys(attrs);
  const values = Object.values(attrs);

  // get a list of key names and value names to match the dynamodb syntax
  const attributeKeyNames = keys.map((k) => '#key_' + k);
  const attributeValueNames = keys.map((k) => ':val_' + k);

  // create individual expressions for each attribute that needs to be updated
  const expressions = attributeValueNames.map((attr, i) => {
    return `${attributeKeyNames[i]} = ${attr}`;
  });

  // add the SET keyword to the beginning of the string
  // and join all the expressions with a comma
  const UpdateExpression = 'SET ' + expressions.join(', ');

  // I use `zipObject()` from lodash https://lodash.com/docs/4.17.15#zipObject
  // it makes an object map from two arrays where the first is the keys and
  // the second is the values
  const ExpressionAttributeValues = _.zipObject(attributeValueNames, values);
  const ExpressionAttributeNames = _.zipObject(attributeKeyNames, keys);

  // now you have all the params
  const params = {
    TableName,
    Key: {
      uuid,
    },
    UpdateExpression,
    ExpressionAttributeValues,
    ExpressionAttributeNames,
  };