How to mock async functions?

[bahrmichael]

I have issues with mocking an async function. I assume that I can return a promise when the mocked method is called, so that the then function can be called on that promise. However when running the test, I get the following error:

  1) LoginSpec
       login should return a valid app token:
     TypeError: Cannot read property 'then' of null
      at Promise (src/login.js:1:11787)
      at new Promise (<anonymous>)
      at LoginService.getToken (src/login.js:1:11319)
      at LoginService.<anonymous> (src/login.js:1:9410)
      ...

Test class:

import {expect} from 'chai';
import {suite, test} from "mocha-typescript";
import {anything, instance, mock, when} from "ts-mockito";
import {LoginService} from '../src/login';
import {RestAccess} from "../src/rest";

@suite
class LoginSpec {

    private restMock = mock(RestAccess);

    @test.only
    public async 'login should return a valid app token'() {
        // given
        const ssoToken = {
            access_token: 'AccessToken',
            refresh_token: 'RefreshToken',
        };

        const promise = new Promise((resolve, reject) => {
            resolve(ssoToken);
        });

        when(this.restMock.fetchJson('url', anything))
            .thenReturn(promise);

        const sut = new LoginService(instance(this.restMock));

        // when
        const event = {
            queryStringParameters: {
                code: 'ssoCode',
            },
        };

        const appToken = await sut.login(event);

        // then
        const decodedToken = this.auth.decodeToken(appToken);
        ...
    }
}

Call to mock:

private getToken(code: string): Promise<any> {
        return new Promise((resolve, reject) => {
            const options = {
                ...
            };

            this.rest.fetchJson('url', options).then(result => {
                console.log('resolve');
                resolve(result);
            }).catch(err => {
                reject(err);
            });
        });
    }

Mocked method:

    public async fetchJson(url: string, options: any) {
        const response = await fetch(url, options);
        const data = await response.json();
        return data;
    }

package.json

{
  "dependencies": {
    "@types/jsonwebtoken": "^7.2.5",
    "@types/mysql": "^2.15.3",
    "@types/node-fetch": "^1.6.7",
    "jsonwebtoken": "^8.2.0",
    "mysql": "^2.15.0",
    "node-fetch": "^2.1.1"
  },
  "devDependencies": {
    "@types/aws-lambda": "0.0.33",
    "@types/chai": "^4.1.2",
    "@types/mocha": "^2.2.48",
    "chai": "^4.1.2",
    "eslint": "^4.18.2",
    "eslint-plugin-mocha": "^4.12.1",
    "mocha": "^5.0.4",
    "mocha-typescript": "^1.1.12",
    "nyc": "^11.4.1",
    "serverless-offline": "^3.18.0",
    "serverless-plugin-typescript": "^1.1.5",
    "ts-mockito": "^2.3.0"
  },
  "scripts": {
    "pretest": "tsc",
    "test": "nyc mocha",
    "watch": "mocha-typescript-watch",
    "prepare": "tsc"
  }
}

[NagRock]

Hi @bahrmichael
First of all you can use thenResolve("value") functionality made by @johanblumenberg https://github.com/NagRock/ts-mockito#resolving--rejecting-promises without manually handling promises.

And the line of code I think makes problem in your example is when(this.restMock.fetchJson('url', anything)).thenReturn(promise);

You have forgotten brackets while using anything matcher" anything => anything()

You can change this line to: when(this.restMock.fetchJson('url', anything()).thenResolve(ssoToken)

[bahrmichael]

anything() did it. JavaScript is not my native language :/ Thank you very much!