What are resolvers? Explain the resolver chain and its arguments.

Question

IT Interview Practice · Accepted Answer

A **resolver** is a function that produces the value for a single field in the schema. GraphQL walks the query field by field and calls the matching resolver for each one — that walk is the **resolver chain**.

## The four resolver arguments

Every resolver receives `(parent, args, context, info)`:

- **parent** — the value returned by the resolver one level up (e.g. the `User` object when resolving `user.posts`).
- **args** — the field's arguments from the query (`{ id: "1" }`).
- **context** — a per-request object shared by all resolvers; holds auth info, the DB client, loaders.
- **info** — the AST and execution details (rarely needed).

```js
const resolvers = {
  Query: {
    // top-level: parent is undefined; read args, use context for data access
    user: (_parent, args, ctx) => ctx.db.users.findById(args.id),
  },
  User: {
    // runs for EACH user; parent is the resolved user object
    posts: (parent, _args, ctx) => ctx.db.posts.findByAuthor(parent.id),
    // no resolver needed for 'name' -> the default resolver reads parent.name
  },
};
```

If a field has no explicit resolver, GraphQL uses a **default resolver** that just reads `parent[fieldName]`. That is why scalar fields on a plain object usually need no code.

## Why it matters

Understanding that resolvers run **per field, top-down, following the parent chain** explains both how nested data is assembled and why naive nested resolvers cause the N+1 problem — the natural follow-up interview question.

The four resolver arguments

Why it matters

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