Node / Sinon - Stub function in library or module (node-postgres)

Question

I'm trying to test a function that queries the database using pg module, here is how I'm using it:

const { Pool } = require('pg');
const { liveDB } = require('../config/db');
const pool = new Pool(liveDB);

exports.query = async (query) => {
    const client = await pool.connect();

    try {
        var result = await client.query(query);
        console.log('result from db.query', result);
        return result;
    } catch (err) {
        console.log('ERROR in db.query')
        console.error(err);
        throw err;
    } finally {
        console.log('Releasing client');
        await client.release();
    }
};

Normally, I would stub a function like this (db.saveUser is a fake function here, but it does get stubbed correctly):

var stub = sinon.stub(db, 'saveUser').callsFake(() => { return 'Saved from stub' });

This however, is not working on the pg module, I tried stubbing the constructor, Pool, .connect, .release and even the entire module, but nothing seems to work for some reason.

PS: I tried changing const to var on all variables as well because I thought it was the cause, same results. I also tried promises in the stub, in several ways, no change.

alexmac · Accepted Answer · 2017-09-08 21:07:41Z

1

Create a new function which returns a pg client and use it in query function:

// db.js

exports.getPgClient = () => {
  return pool.connect();
};

exports.query = async (query) => {
  const client = await exports.getPgClient();
  ...
};

In your test, create a stub for getPgClient function and return a fake client with stubbed connect and release methods:

// test.js

let fakeClient = {
  connect() => { do something or stub this method },
  release() => { do something or stub this method }
};

before() => {
  sinon.stub(db, 'getPgClient').callsFake(() => Promise.resolve(fakeClient));
});

Note: I return a fulfilled promise in stubbed getPgClient function, because original function returns a Promise too.

answered Sep 8, 2017 at 21:07

alexmac

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4 Comments

C.Unbay Over a year ago

sinon.stub() returns a promise ? @alexmac

alexmac Over a year ago

Mankind1023 Over a year ago

Is there a reason why stuff in a library can't be replaced like a normal module? It's fine for a small project but I would rather not have all the extra code / expose some functions that really should not be exported.

alexmac Over a year ago

You have three solutions to solve the problem with testing: 1. As described in the answer, create a new exported function, return created by pg object there and create a mock for this object in test. 2. Use special modules to access external (private) module objects, for example rewire. 3. Create a mock object for pg module completely. In this case you need to think how to require this module. Maybe add if statement, validate process.env,NODE_ENV and require needed module, or maybe create a service which will return needed module.

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