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Correct typescript async await syntax with typed arrow functions

Similar question for vanilla JS here. I however want to use async await with typescript function as below.
Javascript
const foo = async () => {
// do something
}
Initial attempt
export const fetchAirports = async (): Airport[] => {
//More code
return airports;
};
but i get an error ts(1055) Type 'Airport[]' is not a valid async function return type in ES5/ES3 because it does not refer to a Promise-compatible constructor value. which to me sounds like a lot of jargon. Please remember I have to specify a return type in the function declaration otherwise it nullifies the need for typescript in the first place.

Try this
export const fetchAirports = async (): Promise<Airport[]> => {
//More code
return airports;
}
Or
export const fetchAirports = async (): Promise<Array<Airport>> => {
//More code
return airports;
}

When you use async function, it wraps return value in Promise, so instead Airport[] you should wrap it in Promise like this: Promise<Airport[]>

proper import/export and binding for ESM/Node

I have some pre-ES6 code that was working fine with require-type syntax, and am trying to port this to ESM friendly methodology. I have much of the js to mjs coding ported, and am working on a dynamic file loading block, that is giving me fits. I've gotten through enough googling and debugging to realize my issue is now with the syntax of the export function in the "events" file, which I'm using default syntax to support.
First the calling file:
async function eventLoad(eventDir = './events/')
{
await fs.readdir(`./events/`,function (err, files){
if (err)
{
console.error(`${error}: Error loading event: ${err}`);
return;
}
else
{
const events = files.filter(file=>file.endsWith('.mjs'));
for( const file of events)
{
console.log(`event file name is ${file}`);
const {default: event} = import(join(`./events/`,`${file}`));
const eventName = file.split('.')[0];
dBot.on(eventName, event.bind(null,dBot));
console.log(`${success} Loaded event ${eventName}`);
}
}
})
}
eventLoad();
// please ignore the missing promise on the import...I'll be adding it shortly. :)
I get an error:
TypeError: Cannot read property 'bind' of undefined
my export in the other file declares like this:
export default async (client, message) =>
{
...
knowing from googled issues that much of the likelihood rests in the improper definition in the file that is exporting the fuction, I tried playing with the syntax:
async function ProcessMessages (client, message)
{
...
}
export {ProcessMessages as default };
but alas, no help. I'm sure the issue is properly handling the export syntax, but I'm learning this as I go, and would appreciate any help you could provide, thanks!!!
Update: (based on solution, here is my code changes in case anyone else wants to leverage)
async function eventLoad(eventDir = './events/')
{
let files;
try {
files = await promiseBasedReaddir(`${eventDir}`);
}
catch (err) {
console.error(`${error}: Error loading event: ${err}`);
return;
}
const events = files.filter(file=>file.endsWith('.mjs'));
for( const file of events)
{
console.log(`event file name is ${file}`);
import(`${eventDir}${file}`)
.then(function( {default: event} ){
const eventName = file.split('.')[0];
dBot.on(eventName, event.bind(null,dBot));
console.log(`${success} Loaded event ${eventName}`);
})
.catch(function(err){
console.error(`${error}: Error loading event: ${err}`);
return;
})
}
}
eventLoad();

On this line, you do a dynamic import:
const {default: event} = import(join(`./events/`,`${file}`));
import will always return a Promise. The Promise object doesn't have a .bind method, only functions do. You need to await or .then() import()'s return value. So you can't "ignore the missing promise on the import".
A second issue:
await fs.readdir(`./events/`, ...´
fs.readdir doesn't return a promise, so await-ing it is useless. It is functionally the same as await undefined;. The outer function will return before the callback has finished.
If you use a readdir function that returns a promise, you can simplify things a lot.
import { promises as fsPromises } from 'fs';
const { readdir: promiseBasedReaddir } = fsPromises;
// OR alternatively:
import { readdir } from 'fs';
import { promisify} from 'util';
const promiseBasedReaddir = promisify(readdir);
async function eventLoad(eventDir = './events/')
{
let files;
try {
files = await promiseBasedReaddir(`./events/`);
} catch (err) {
console.error(`${error}: Error loading event: ${err}`);
return;
}
const events = files.filter(file=>file.endsWith('.mjs'));
for( const file of events)
{
console.log(`event file name is ${file}`);
const {default: event} = await import(join(`./events/`,`${file}`));
const eventName = file.split('.')[0];
dBot.on(eventName, event.bind(null,dBot));
console.log(`${success} Loaded event ${eventName}`);
}
}
Promisify will transform a callback-using function to a promise-returning function. The fs module has promise-based functions, so you don't need to use Promisify if you just import those.

Jest testing context / spy on mocked variables created outside of functions (class level) Postmark

I'm trying to do some testing in Jest but getting stuck with a mock/spy. I've managed to get the test working but only by changing my implementation (which I feel dirty about).
Here's the test:
import * as postmark from 'postmark';
jest.mock('postmark');
const mockGetServers = jest.fn();
const AccountClient = jest.fn(() => {
return {
getServers: mockGetServers
};
});
postmark.AccountClient = AccountClient;
import accountApi from './account-api';
describe('account-api', () => {
describe('listServers', () => {
it('calls postmark listServers', async () => {
await accountApi.listServers();
expect(mockGetServers).toHaveBeenCalledTimes(1);
});
});
});
Here's the working implementation:
import * as postmark from 'postmark';
const accountToken = 'some-token-number';
const listServers = async () => {
try {
const accountClient = postmark.AccountClient(accountToken);
const servers = await accountClient.getServers();
return servers;
} catch (e) {
console.log('ERROR', e);
}
};
export default {
listServers
}
Here's the original implementation:
import * as postmark from 'postmark';
const accountToken = 'some-token-number';
const accountClient = postmark.AccountClient(accountToken);
const listServers = async () => {
try {
const servers = await accountClient.getServers();
return servers;
} catch (e) {
console.log('ERROR', e);
}
};
export default {
listServers
}
The only change is where in the code the accountClient is created (either inside or outside of the listServers function). The original implementation would complete and jest would report the mock hadn't been called.
I'm stumped as to why this doesn't work to start with and guessing it's something to do with context of the mock. Am I missing something about the way jest works under the hood? As the implementation of accountApi will have more functions all using the same client it makes sense to create one for all functions rather than per function. Creating it per function doesn't sit right with me.
What is different about the way I have created the accountClient that means the mock can be spied on in the test? Is there a way I can mock (and spy on) the object that is created at class level not at function level?
Thanks

Am I missing something about the way jest works under the hood?
Two things to note:
ES6 import calls are hoisted to the top of the current scope
babel-jest hoists calls to jest.mock to the top of their code block (above everything including any ES6 import calls in the block)
What is different about the way I have created the accountClient that means the mock can be spied on in the test?
In both cases this runs first:
jest.mock('postmark');
...which will auto-mock the postmark module.
Then this runs:
import accountApi from './account-api';
In the original implementation this line runs:
const accountClient = postmark.AccountClient(accountToken);
...which captures the result of calling postmark.AccountClient and saves it in accountClient. The auto-mock of postmark will have stubbed AccountClient with a mock function that returns undefined, so accountClient will be set to undefined.
In both cases the test code now starts running which sets up the mock for postmark.AccountClient.
Then during the test this line runs:
await accountApi.listServers();
In the original implementation that call ends up running this:
const servers = await accountClient.getServers();
...which drops to the catch since accountClient is undefined, the error is logged, and the test continues until it fails on this line:
expect(mockGetServers).toHaveBeenCalledTimes(1);
...since mockGetServers was never called.
On the other hand, in the working implementation this runs:
const accountClient = postmark.AccountClient(accountToken);
const servers = await accountClient.getServers();
...and since postmark is mocked by this point it uses the mock and the test passes.
Is there a way I can mock (and spy on) the object that is created at class level not at function level?
Yes.
Because the original implementation captures the result of calling postmark.AccountClient as soon as it is imported, you just have to make sure your mock is set up before you import the original implementation.
One of the easiest ways to do that is to set up your mock with a module factory during the call to jest.mock since it gets hoisted and runs first.
Here is an updated test that works with the original implementation:
import * as postmark from 'postmark';
jest.mock('postmark', () => { // use a module factory
const mockGetServers = jest.fn();
const AccountClient = jest.fn(() => {
return {
getServers: mockGetServers // NOTE: this returns the same mockGetServers every time
};
});
return {
AccountClient
}
});
import accountApi from './account-api';
describe('account-api', () => {
describe('listServers', () => {
it('calls postmark listServers', async () => {
await accountApi.listServers();
const mockGetServers = postmark.AccountClient().getServers; // get mockGetServers
expect(mockGetServers).toHaveBeenCalledTimes(1); // Success!
});
});
});

I think you might want to look at proxyquire.
import * as postmark from 'postmark';
import * as proxyquire from 'proxyquire';
jest.mock('postmark');
const mockGetServers = jest.fn();
const AccountClient = jest.fn(() => {
return {
getServers: mockGetServers
};
});
postmark.AccountClient = AccountClient;
import accountApi from proxyquire('./account-api', postmark);
describe('account-api', () => {
describe('listServers', () => {
it('calls postmark listServers', async () => {
await accountApi.listServers();
expect(mockGetServers).toHaveBeenCalledTimes(1);
});
});
});
Note that I have not tested this implementation; tweaking may be required.

What is the state of the art for testing/mocking functions within a module in 2018?

I have a module, for the purposes of learning testing, that looks like this:
api.js
import axios from "axios";
const BASE_URL = "https://jsonplaceholder.typicode.com/";
const URI_USERS = 'users/';
export async function makeApiCall(uri) {
try {
const response = await axios(BASE_URL + uri);
return response.data;
} catch (err) {
throw err.message;
}
}
export async function fetchUsers() {
return makeApiCall(URI_USERS);
}
export async function fetchUser(id) {
return makeApiCall(URI_USERS + id);
}
export async function fetchUserStrings(...ids) {
const users = await Promise.all(ids.map(id => fetchUser(id)));
return users.map(user => parseUser(user));
}
export function parseUser(user) {
return `${user.name}:${user.username}`;
}
Pretty straight forward stuff.
Now I want to test that fetchUserStrings method, and to do that I want to mock/spy on both fetchUser and parseUser. At the same time - I don't want the behaviour of parseUser to stay mocked - for when I'm actually testing that.
I run in the problem that it seems that it is not possible to mock/spy on functions within the same module.
Here are the resources I've read about it:
How to mock a specific module function? Jest github issue. (100+ thumbs up).
where we're told:
Supporting the above by mocking a function after requiring a module is impossible in JavaScript – there is (almost) no way to retrieve the binding that foo refers to and modify it.
The way that jest-mock works is that it runs the module code in isolation and then retrieves the metadata of a module and creates mock functions. Again, in this case it won't have any way to modify the local binding of foo.
Refer to the functions via an object
The solution he proposes is ES5 - but the modern equivalent is described in this blog post:
https://luetkemj.github.io/170421/mocking-modules-in-jest/
Where, instead of calling my functions directly, I refer to them via an object like:
api.js
async function makeApiCall(uri) {
try {
const response = await axios(BASE_URL + uri);
return response.data;
} catch (err) {
throw err.message;
}
}
async function fetchUsers() {
return lib.makeApiCall(URI_USERS);
}
async function fetchUser(id) {
return lib.makeApiCall(URI_USERS + id);
}
async function fetchUserStrings(...ids) {
const users = await Promise.all(ids.map(id => lib.fetchUser(id)));
return users.map(user => lib.parseUser(user));
}
function parseUser(user) {
return `${user.name}:${user.username}`;
}
const lib = {
makeApiCall,
fetchUsers,
fetchUser,
fetchUserStrings,
parseUser
};
export default lib;
Other posts that suggest this solution:
https://groups.google.com/forum/#!topic/sinonjs/bPZYl6jjMdg
https://stackoverflow.com/a/45288360/1068446
And this one seems to be a variant of the same idea:
https://stackoverflow.com/a/47976589/1068446
Break the object into modules
An alternative, is that I would break my module up, such that I'm never calling functions directly within each other.
eg.
api.js
import axios from "axios";
const BASE_URL = "https://jsonplaceholder.typicode.com/";
export async function makeApiCall(uri) {
try {
const response = await axios(BASE_URL + uri);
return response.data;
} catch (err) {
throw err.message;
}
}
user-api.js
import {makeApiCall} from "./api";
export async function fetchUsers() {
return makeApiCall(URI_USERS);
}
export async function fetchUser(id) {
return makeApiCall(URI_USERS + id);
}
user-service.js
import {fetchUser} from "./user-api.js";
import {parseUser} from "./user-parser.js";
export async function fetchUserStrings(...ids) {
const users = await Promise.all(ids.map(id => lib.fetchUser(id)));
return ids.map(user => lib.parseUser(user));
}
user-parser.js
export function parseUser(user) {
return `${user.name}:${user.username}`;
}
And that way I can mock the dependency modules when I'm testing the dependant module, no worries.
But I'm not sure that breaking up the modules like this is even feasible - I imagine that there might be a circumstance where you have circular dependencies.
There are some alternatives:
Dependency injection in the function:
https://stackoverflow.com/a/47804180/1068446
This one looks ugly as though, imo.
Use babel-rewire plugin
https://stackoverflow.com/a/52725067/1068446
I have to admit - I haven't looked at this much.
Split your test into multiple files
Am investigating this one now.
My question: This is all quite a frustrating and fiddly way of testing - is there a standard, nice and easy, way people are writing unit tests in 2018, that specifically solve this issue?

As you've already discovered attempting to directly test an ES6 module is extremely painful. In your situation it sounds like you are transpiling the ES6 module rather than testing it directly, which would likely generate code that looks something like this:
async function makeApiCall(uri) {
...
}
module.exports.makeApiCall = makeApiCall;
Since the other methods are calling makeApiCall directly, rather than the export, even if you tried to mock the export nothing would happen. As it stands ES6 module exports are immutable, so even if you did not transpile the module you would likely still have issues.
Attaching everything to a "lib" object is probably the easiest way to get going, but it feels like a hack, not a solution. Alternatively using a library that can rewire the module is a potential solution, but its extremely hokey and in my opinion it smells. Usually when you're running into this type of code smell you have a design problem.
Splitting up the modules into tiny pieces feels like a poor mans dependency injection, and as you've stated you'll likely run into issues quickly. Real dependency injection is probably the most robust solution, but it's something you need to build from the ground up, it's not something that you can just plug into an existing project and expect to have things working immediately.
My suggestion? Create classes and use them for testing instead, then just make the module a thin wrapper over an instance of the class. Since you're using a class you'll always be referencing your method calls using a centralized object (the this object) which will allow you to mock out the things you need. Using a class will also give you an opportunity to inject data when you construct the class, giving you extremely fine grained control in your tests.
Let's refactor your api module to use a class:
import axios from 'axios';
export class ApiClient {
constructor({baseUrl, client}) {
this.baseUrl = baseUrl;
this.client = client;
}
async makeApiCall(uri) {
try {
const response = await this.client(`${this.baseUrl}${uri}`);
return response.data;
} catch (err) {
throw err.message;
}
}
async fetchUsers() {
return this.makeApiCall('/users');
}
async fetchUser(id) {
return this.makeApiCall(`/users/${id}`);
}
async fetchUserStrings(...ids) {
const users = await Promise.all(ids.map(id => this.fetchUser(id)));
return users.map(user => this.parseUser(user));
}
parseUser(user) {
return `${user.name}:${user.username}`;
}
}
export default new ApiClient({
url: "https://jsonplaceholder.typicode.com/",
client: axios
});
Now lets create some tests for the ApiClient class:
import {ApiClient} from './api';
describe('api tests', () => {
let api;
beforeEach(() => {
api = new ApiClient({
baseUrl: 'http://test.com',
client: jest.fn()
});
});
it('makeApiCall should use client', async () => {
const response = {data: []};
api.client.mockResolvedValue(response);
const value = await api.makeApiCall('/foo');
expect(api.client).toHaveBeenCalledWith('http://test.com/foo');
expect(value).toBe(response.data);
});
it('fetchUsers should call makeApiCall', async () => {
const value = [];
jest.spyOn(api, 'makeApiCall').mockResolvedValue(value);
const users = await api.fetchUsers();
expect(api.makeApiCall).toHaveBeenCalledWith('/users');
expect(users).toBe(value);
});
});
I should note that I have not tested if the provided code works, but hopefully the concept is clear enough.

exporting after promise finishes

I would like to export a class which initial state depends on a value returned from a Promise in another module i cannot modify.
Here's the code:
let e = true;
APromiseFromAnotherModule()
.then(value => return value;);
export default class E {
constructor() {
if (e) {
//...
} else {
//...
}
}
}
I also tried with async/await encapsulating the Promise into an async function like this:
let e = true;
getInitialValue = async () => {
return await APromiseFromAnotherModule()
.then(value => e = value;);
};
e = getInitialValue();
export default class E {
constructor() {
if (e) {
//...
} else {
//...
}
}
}
But it doesn't make sense because that one is an async function so obviously it doesn't work.
What am I missing?

module exports are done synchronously. So, they cannot depend upon the results of an asynchronous operation.
And, await only works inside a function. It doesn't actually block the containing function (the containing function returns a promise) so that won't help you make an async operation into a synchronous one either.
The usual ways to deal with a module that uses some async code in its setup is to either export a promise and have the calling code use .then() on the promise or to initialize the module with a constructor function that returns a promise.
The code is only pseudo code so it's hard to tell exactly what your real problem is to show you specific code for your situation.
As an example. If you want to export a class definition, but don't want the class definition used until some async code has completed, you can do something like this:
// do async initialization and keep promise
let e;
const p = APromiseFromAnotherModule().then(val => e = val);
class E {
constructor() {
if (e) {
//...
} else {
//...
}
}
};
// export constructor function
export default function() {
return p.then(e => {
// after async initialization is done, resolve with class E
return E;
});
}
The caller could then use it like this:
import init from 'myModule';
init().then(E => {
// put code here that uses E
}).catch(err => {
console.log(err);
// handle error here
});
This solves several issues:
Asynchronous initialization is started immediately upon module loading.
class E is not made available to the caller until the async initialization is done so it can't be used until its ready
The same class E is used for all users of the module
The async initialization is cached so it's only done once
Edit in 2023. Modern nodejs versions when using ESM modules have top level await so it is possible to await an asynchronous result before your exports.

I understand that #jfriend00's answer works fine, but in my case, there is not just one block of code that depends on the foreign async function's completion.
In my app, configuration loading is asynchronous, so the server must wait for the configs to load before starting. Since there are other files (like routes) that compose the server that also need access to the configs, I would have to reluctantly call .then() in each other file.
Here is how I was able to do it with require statements instead of export.
config.js
Module that can be required by other modules in order to gain access to the global configs.
module.exports.loadAllConfigs = async () => {
const appConfigs = await AsyncConfigLibrary.load();
module.exports.CONFIG = appConfigs;
};
server.js
Main file for Node application that makes use of other modules that require access to global configs.
const { loadAllConfigs } = require('./modules/config');
loadAllConfigs()
.then(() => {
const { CONFIG } = require('./modules/config');
/* import modules */
const auth = require('./modules/auth');
};
auth.js
Module used by server.js that requires access to configs.
const { CONFIG } = require('./config');
const cookieSecret = CONFIG.secretItem;
Therefore, as long as the CONFIG property is set in config.js before any of the other modules attempt to access it, that is, before the modules are require'd, then the single .then() in server.js is sufficient.