Example code for creating an Apollo Link and corresponding test

OperationHistoryLink.test.js

import { ApolloLink, Observable, execute } from '@apollo/client';

// stateful link
class OperationHistoryLink extends ApolloLink {
  operationHistory = [];

  request(operation, forward) {
    this.operationHistory.push(operation);
    return forward(operation);
  }
}

// stateless, terminating link
const mockLink = new ApolloLink((operation) => {
  const context = operation.getContext();
  const { mockError, mockResponse } = context || {};

  return new Observable((observer) => {
    if (mockError) {
      observer.error(mockError);
      return;
    }

    if (mockResponse) {
      observer.next({ data: mockResponse });
      observer.complete();
      return;
    }

    observer.error(
      new Error('[MockLink]: You must provide mockError or mockResponse'),
    );
  });
});

describe('OperationHistoryLink', () => {
  it('adds operation to the operationHistory array on success', () => {
    const operationHistoryLink = new OperationHistoryLink();
    // create a link chain with our link-to-test and the mockLink
    const composedLink = operationHistoryLink.concat(mockLink);

    return new Promise((resolve) => {
      const operation = {
        query: 'some query',
        context: {
          // test a successful response
          mockResponse: { foo: 'bar' },
        },
      };

      execute(composedLink, operation).subscribe({
        next: ({ data }) => {
          expect(data.foo).toBe('bar'); // mockResponse is handled by mockLink
          expect(operationHistoryLink.operationHistory).toHaveLength(1);
          expect(operationHistoryLink.operationHistory[0].query).toEqual(
            operation.query,
          );
          resolve();
        },
        error: () => {
          // optionally reject or throw here, we shouldn't get an error
        },
        complete: () => {},
      });
    });
  });
});