update attributes in a dynamodb record - node.js

Question

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

Answer 1

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,
};

Answer 2

Lucas D's answer works great. A couple of things to keep in mind though:
If you're sending an object with a unique key into your update function, you must remove that key before mapping your expression arrays or it will be included in the query and will get an error:
const newObject = {...originalObject}
delete newObject.unique_key
If you can't or don't want to use Lodash, you can use Object.assign to map your keys/values arrays:
const ExpressionAttributeValues = Object.assign(...attributeValueNames.map((k, i) => ({[k]: values[i]})));
const ExpressionAttributeNames = Object.assign(...attributeKeyNames.map((k, i) => ({[k]: keys[i]})));