Hide console logging for passing tests and show it for failures with node:test

Coming from pytest, I was surprised to find that most javascript testing frameworks do no capturing & filtering of console.logs. Which means that you often end up with a noisy output even when a test passes. In the example below it's made worse by using console.error.

I've been looking for a solution to this for sometime, and others have too, this jest issue from 2018 sums it up pretty well. There are two common solutions proposed to this problem. One is to mock the implementation of console.* but this means you loose important context in the event that a test does fail and you need to review the logs.

The other suggestion is to use a custom reporter, I've tried the one that is suggested in the jest issue but it'll log all console.logs from the entire test suite if one test fails, which is better but still not what you ideally want.

A built in test runner was introduced in node.js V18 which I feel is likely going to become the defacto as it matures. I noticed they had some docs on custom reporters and so I thought I'd take a look at finding a solution to this age old problem.

The node.js runner can provide a TAP output. Which I thought would be easy to find a good reporter that hides console.logs, but none that I looked at were well maintained or feature complete and so I'd have to build my own. I quite liked the output of the default node.js spec reporter and so I abandondoned the TAP reported and set about using that as my starting point.

I ended up building a custom reporter that delegates all behaviour to the builtin spec reporter, except for the test:stderror and test:stdout events where I capture the logs and then only stream them on if a test fails, it ended up being pretty simple once I figure out how Trasform Streams worked.

import { spec } from "node:test/reporters";
import { Transform } from 'node:stream';

class Reporter extends Transform {
  constructor() {
    super({ writableObjectMode: true })
    this.specReporter = new spec()
    this.logs = []
  }

  addToFailedLogs = (_, data) => {
    this.logs.push(data)
  }

  _transform(event, encoding, callback) {
    switch (event.type) {
      case 'test:stderr':
      case 'test:stdout':
        // Any output from the spec reporter is pushed to this.logs
        this.logs.push(this.specReporter._transform(event, encoding, this.addToFailedLogs));
        callback(null)
        break;
      case 'test:pass':
        this.specReporter._transform(event, encoding, callback);
        this.logs = [];
        break;
      case 'test:fail':
        this.specReporter._transform(event, encoding, this.addToFailedLogs);
        callback(null, this.logs.join(''))
        this.logs = []
        break;
      default:
        this.specReporter._transform(event, encoding, callback);
        break;
    }

  }

  _flush(callback) {
    this.specReporter._flush(callback)
  }
}

export default new Reporter()

gist link

To use this reporter you simply need to run your tests with the --test-reporter flag. For example:

node node --test-reporter ./reporter.mjs --test ./tests/*.mjs

Hope this helps you if you are looking for a little better testing ergonomics in javascript.