The shape of a resolver

5. The shape of a resolver

Our RESTDataSource is ready to use, but we don't yet have any resolvers to use it!

A resolver's mission is to populate the data for a field in your schema. Your mission is to implement those resolvers!

What exactly is a resolver? A resolver is a function. It has the same name as the field that it populates data for. It can fetch data from any data source, then transforms that data into the shape your client requires.

Hand-drawn illustration depicting a resolver function retrieving data from data-land

In server/src/ we'll start by creating a resolvers.js file.

In that file, we'll declare a resolvers constant, assigning it an empty object for now. Let's export it, because we'll need it in our server config options.

const resolvers = {};

module.exports = resolvers;

export const resolvers = {};

Our resolvers object's keys will correspond to our schema's types and fields.

To create a resolver for the tracksForHome field, we'll first add a Query key to our resolvers object. The value of that key will be another object that contains the tracksForHome key.

This tracksForHome key is where we'll define our resolver function for the corresponding field.

While we're here, let's add a comment above the resolver to clarify what it does.

Inside the resolvers object:

Query: {
  // returns an array of Tracks that will be used to populate
  // the homepage grid of our web client
  tracksForHome: () => {},
}

Which of the following are true about resolvers?

Each resolver corresponds to a different data source.A resolver transforms data into the shape a query requires.A resolver is a function that's responsible for populating the data for a single field in your schema.

How will our resolvers interact with our data source? This is where contextValue comes in. Resolver functions have a specific signature with four optional parameters: parent, args, contextValue, and info.

Hand-drawn illustration depicting a resolver function signature with its four parameters

tracksForHome: (parent, args, contextValue, info) => {},

Let's go over each parameter briefly to understand what they're responsible for:

parent:
parent is the returned value of the resolver for this field's parent. This will be useful when dealing with resolver chains.
args:
args is an object that contains all GraphQL arguments that were provided for the field by the GraphQL operation. When querying for a specific item (such as a specific track instead of all tracks), in client-land we'll make a query with an id argument that will be accessible via this args parameter in server-land. We'll cover this further in Lift-off III.
contextValue:
contextValue is an object shared across all resolvers that are executing for a particular operation. The resolver needs this argument to share state, like authentication information, a database connection, or in our case the RESTDataSource.
info:
info contains information about the operation's execution state, including the field name, the path to the field from the root, and more. It's not used as frequently as the others, but it can be useful for more advanced actions like setting cache policies at the resolver level.

What is the contextValue parameter useful for?

Setting cache policies at the resolver levelAccessing GraphQL arguments provided for a fieldSharing information across all resolvers for an operationAccessing data sources from a resolver

Code Challenge!

Write an empty resolver function for the field spaceCats with all four parameters as described above. The function should not return anything. Use arrow function syntax.

In this course, we'll be focusing on contextValue, the third positional argument that's passed to every resolver function. Let's dig into it.

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