Pothos GraphQL


Defining Union Types

Union types are defined with a list of object types:

const builder = new SchemaBuilder<{
  Objects: {
    GiraffeStringFact: { factKind: 'string'; fact: string };
    GiraffeNumericFact: { factKind: 'number'; fact: string; value: number };

builder.objectType('GiraffeStringFact', {
  fields: (t) => ({
    fact: t.exposeString('fact', {}),

const GiraffeNumericFact = builder.objectType('GiraffeNumericFact', {
  fields: (t) => ({
    fact: t.exposeString('fact', {}),
    value: t.exposeFloat('value', {}),

const GiraffeFact = builder.unionType('GiraffeFact', {
  types: ['GiraffeStringFact', GiraffeNumericFact],
  resolveType: (fact) => {
    switch (fact.factKind) {
      case 'number':
        return GiraffeNumericFact;
      case 'string':
        return 'GiraffeStringFact';

The types array can either contain Object type names defined in SchemaTypes, or and Object Ref created by object type. builder.objectType, builder.objectRef or other method, or a class that was used to implement an object type.

The resolveType function will be called with each item returned by a field that returns the unionType, and is used to determine which concrete the value corresponds to. It is usually good to have a shared property you can use to differentiate your union members.

Using Union Types

builder.queryField('giraffeFacts', (t) =>
    type: [GiraffeFact],
    resolve: () => {
      const fact1 = {
        factKind: 'string',
        fact: 'A giraffe’s spots are much like human fingerprints. No two individual giraffes have exactly the same pattern',
      } as const;

      const fact2 = {
        factKind: 'number',
        fact: 'Top speed (MPH)',
        value: 35,
      } as const;

      return [fact1, fact2];