Migrating from VTL to JavaScript

Amazon AppSync allows you to write your business logic for your resolvers and functions using VTL or JavaScript. With both languages, you write logic that instructs the Amazon AppSync service on how to interact with your data sources. With VTL, you write mapping templates that must evaluate to a valid JSON-encoded string. With JavaScript, you write request and response handlers that return objects. You don't return a JSON-encoded string.

For example, take the following VTL mapping template to get an Amazon DynamoDB item:

{
    "operation": "GetItem",
    "key": {
        "id": $util.dynamodb.toDynamoDBJson($ctx.args.id),
    }
}

The utility $util.dynamodb.toDynamoDBJson returns a JSON-encoded string. If $ctx.args.id is set to <id>, the template evaluates to a valid JSON-encoded string:

{
    "operation": "GetItem",
    "key": {
        "id": {"S": "<id>"},
    }
}

When working with JavaScript, you do not need to print out raw JSON-encoded strings within your code, and using a utility like toDynamoDBJson is not needed. An equivalent example of the mapping template above is:

import { util } from '@aws-appsync/utils';
export function request(ctx) {
  return {
    operation: 'GetItem',
    key: {id: util.dynamodb.toDynamoDB(ctx.args.id)}
  };
}

An alternative is to use util.dynamodb.toMapValues, which is the recommended approach to handle an object of values:

import { util } from '@aws-appsync/utils';
export function request(ctx) {
  return {
    operation: 'GetItem',
    key: util.dynamodb.toMapValues({ id: ctx.args.id }),
  };
}

This evaluates to:

{
  "operation": "GetItem",
  "key": {
    "id": {
      "S": "<id>"
    }
  }
}

import * as ddb from '@aws-appsync/utils/dynamodb'

export function request(ctx) {
	ddb.get({ key: { id: ctx.args.id } })
}

As another example, take the following mapping template to put an item in an Amazon DynamoDB data source:

{
    "operation" : "PutItem",
    "key" : {
        "id": $util.dynamodb.toDynamoDBJson($util.autoId()),
    },
    "attributeValues" : $util.dynamodb.toMapValuesJson($ctx.args)
}

When evaluated, this mapping template string must produce a valid JSON-encoded string. When using JavaScript, your code returns the request object directly:

import { util } from '@aws-appsync/utils';
export function request(ctx) {
  const { id = util.autoId(), ...values } = ctx.args;
  return {
    operation: 'PutItem',
    key: util.dynamodb.toMapValues({ id }),
    attributeValues: util.dynamodb.toMapValues(values),
  };
}

which evaluates to:

{
  "operation": "PutItem",
  "key": {
    "id": { "S": "2bff3f05-ff8c-4ed8-92b4-767e29fc4e63" }
  },
  "attributeValues": {
    "firstname": { "S": "Shaggy" },
    "age": { "N": 4 }
  }
}

import { util } from '@aws-appsync/utils'
import * as ddb from '@aws-appsync/utils/dynamodb'

export function request(ctx) {
	const { id = util.autoId(), ...item } = ctx.args
	return ddb.put({ key: { id }, item })
}