Node.js Tips

Node.js Tips — Build Errors, Tests, and Exit Codes

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As with any kind of app, there are difficult issues to solve when we write Node apps. In this article, we’ll look at some solutions to common problems when writing Node apps.

Fix NPM Error on Windows Looking for Tools Version

If we install native Node packages on Windows, we may run into errors that look something like:

MSBUILD : error MSB4132: The tools version "2.0" is unrecognized. Available tools versions are "4.0".

To fix this, we can run:

npm install --global --production windows-build-tools

in admin mode to install the build tools for Windows to compile those packages.

Use a Custom Express Server in Supertest

We have to export out Express server so that it can be tested with Supertest.

For instance, we can write:


const app = express();

module.exports = app;

Then in our test, we can write:

const request = require('supertest');
const app = require('./app');

describe('POST /', () => {
  it('should make request with foo', (done) => {
        'foo': 'bar'
      .end((err, res) => {

We pass in the app which exported into the request function to make a request with our Express app.

Get Query String Params with Koa Router

To get a query string parameter with the Koa router, we can use the ctx.request.query property.

For instance, we can write:

import koaRouter from 'koa-router';

const router = koaRouter({
  prefix: '/foo'

router.get('/', async (ctx) => {

to get the parsed query parameters.

We can also use ctx.query for short:

import koaRouter from 'koa-router';

const router = koaRouter({
  prefix: '/foo'

router.get('/', async (ctx) => {

Accessing the Exit Code and stderr of a System Command

We can access the exit code and stderr of a system command by getting them from the error object.

For instance, if we’re using execSync to run our command, then we can write:

const childProcess = require('child_process');
const cmd = 'some command';

try {
} catch (error) {

We call execSync with a command.

Then we get the error with the catch block.

status has the exit code.

message has error messages.

stderr has the full error output.

stdout has the standard output.

If we use exec , we can get the error from the callback.

const childProcess = require('child_process');
const cmd = 'some command';

child_process.exec(cmd, (err, stdout, stderr) => {
}).on('exit', code => console.log(code))

We can get the stdout and stderr from the callback.

The exit code can be obtained by listening to the exit event.

It takes a callback with the exit code, which is the value of code .

err has the errors encountered.

Node.js server.address().address Returns :: in an Express App

To make server.address return an IP address, we’ve to specify the IP address specifically with listen .

For example, we can write:

const express = require('express');
const app = express();
const server = app.listen(8888, "", () => {
  const { address, port } = server.address();
  console.log(`running at http://${host}:${port}`);

We pass in the IP address as the 2nd argument.

Then server.address will get the host , which has the IP address, and port , which has the port.

Unit Test an Event Being Emitted in Mocha

We can listen to events in our Mocha tests.

For instance, we can write:

it('should emit an event', function(done){

  eventObj.on('event', () => {
    // run assertions...

We call this.timeout to make sure that the test times out with an error if the event isn’t received in 1 second.

Then we can listen to the event event emitted by eventObj and run any assertions we want with it.

After that, we call done to move on to other pieces of code.


We can pass the IP address on listen to get the IP address the app is listening to with server.address() . Errors and exit code for shell commands can be retrieved with exec and execSync . We’ve to install windows-build-tools globally to fix any errors with build native packages. To test our Express app with Supertest, we’ve to import our Express app object and pass it to Supertest’s request function before we make our request.

By John Au-Yeung

Web developer specializing in React, Vue, and front end development.

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