AWS AppSync JavaScript Resolver Skill

Write resolvers using the AppSync JavaScript (APPSYNC_JS) runtime. Resolvers connect GraphQL operations to data sources using familiar JavaScript syntax.

Core Pattern

Every resolver exports a request function and a response function:

export function request(ctx) { // Build the data source request using ctx.args, ctx.identity, ctx.source, etc. return { /* data source request */ }; }

export function response(ctx) { // Transform and return the data source response return ctx.result; }

The ctx (context) object provides:

• ctx.args — GraphQL field arguments

• ctx.result — the data source response (in response handler)

• ctx.error — error from the data source, if any

• ctx.identity — caller identity (Cognito, IAM, etc.)

• ctx.source — parent field value (for nested resolvers)

• ctx.info — field name, parent type, selection set

• ctx.prev.result — result from previous function (pipeline resolvers)

• ctx.stash — shared object across pipeline functions

• ctx.request — request headers

Imports

import { util } from '@aws-appsync/utils'; import * as ddb from '@aws-appsync/utils/dynamodb'; import { sql, select, insert, update, remove, createPgStatement, createMySQLStatement, toJsonObject } from '@aws-appsync/utils/rds';

Only import what you need. The @aws-appsync/utils/dynamodb module provides high-level helpers. The @aws-appsync/utils/rds module provides SQL builders.

Choosing the Right Data Source

Data Source When to Use

DynamoDB NoSQL key-value/document storage — most common

HTTP REST APIs, API Gateway, or AWS service APIs (SNS, Translate, etc.)

Lambda Complex business logic that can't run in APPSYNC_JS

RDS Aurora PostgreSQL/MySQL via Data API

OpenSearch Full-text search, geo queries, analytics

EventBridge Event-driven architectures, publish events

NONE Local operations — subscriptions, transforms, no external call

Read the relevant reference file below for the data source patterns.

Reference Files

Each data source has a dedicated reference with complete code examples. Read the one you need:

• references/dynamodb.md — DynamoDB CRUD, queries, batch operations, scans, pagination

• references/http.md — HTTP requests, API Gateway, SNS, AWS Translate

• references/rds.md — Aurora PostgreSQL/MySQL with the select /sql builders

• references/opensearch.md — Full-text search, geo, pagination

• references/lambda-eventbridge-none.md — Lambda invoke, EventBridge PutEvents, local publish, enhanced subscriptions

• references/pipeline.md — Pipeline resolvers (chaining multiple functions)

APPSYNC_JS Runtime Restrictions

The APPSYNC_JS runtime does NOT support:

• for / while / do...while loops — use .forEach() , .map() , .filter() , .reduce() instead

• Array.sort() — not available; implement merge logic via .forEach() on concatenated arrays

• Arrow functions in array method callbacks — can cause ReferenceError for callback parameters. Always use traditional function expressions: // WRONG — arrow function parameter may be undefined at runtime items.forEach((item) => { doSomething(item) })

// CORRECT — traditional function expression items.forEach(function (item) { doSomething(item) })

• try / catch / finally — use ctx.error checks instead

• async / await — not supported

• Classes, generators, symbols, WeakRef, eval

• returnValues in UpdateItem — not supported; error: Unsupported element '$[returnValues]' . ctx.result after UpdateItem is empty (no updated attributes). Use optimistic counts on the frontend, or add a follow-up GetItem pipeline function to read the updated value

Error Handling

Two patterns depending on severity:

// Hard error — stops execution and returns error to client if (ctx.error) { util.error(ctx.error.message, ctx.error.type); }

// Soft error — appends error but still returns data util.appendError(body, statusCode);

Common Utilities

util.autoId() // Generate a unique ID util.time.nowISO8601() // Current timestamp util.dynamodb.toMapValues({ id }) // Convert JS object to DynamoDB attribute map util.transform.toSubscriptionFilter() // Build subscription filters util.xml.toMap(body) // Parse XML to object util.urlEncode(str) // URL-encode a string util.http.copyHeaders(ctx.request.headers) // Copy request headers

Development Setup

Install the AppSync utilities for type checking and editor support:

npm install @aws-appsync/utils @aws-appsync/eslint-plugin

Use JSDoc type annotations for better IDE support:

/** @param {import('@aws-appsync/utils').Context<{id: string}>} ctx */ export function request(ctx) { ... }