assert-log

Any easy way to unit-test callbacks

If you need to unit-test some code that calls callbacks, you could use a mocking library like sinon.js, but there is a simpler technique. Just write your own callbacks, but have them send their output to a log. When the test is over, verify that the log matches your expectation. For example, suppose you want to unit-test this thing:

class ExampleCounter {
  on(event, callback) { /* ... */ }
  increment() { /* ... */ }
  decrement() { /* ... */ }
}

Here is what the test might look like:

import { makeAssertLog } from 'assert-log'

const log = makeAssertLog()

// Set up the callbacks:
const counter = new ExampleCounter()
counter.on('count', count => log('count is', count))
counter.on('error', error => log(error))

// Generate some events:
counter.increment()
log.assert('count is 1')

counter.decrement()
log.assert('count is 0')

counter.decrement()
log.assert('Error: Count cannot be negative')

The log function records a log entry. It accepts multiple arguments, which it concatenates together to make a single string.

The assert method verifies that that log contains the given strings in any order, and then clears the log.

Setup

Install:

yarn add --dev assert-log
# or:
npm install --save-dev assert-log

Import:

import { makeAssertLog } from 'assert-log'
// or:
const { makeAssertLog } = require('assert-log')

Advanced usage

If you need more control than assert provides, the log also has a read method. This method removes log entries and returns them. It accepts the maximum number of entries to return, but this is optional. It returns an array of strings. The returned array also has an assert method for convenience:

// Read and check the first entry:
log.read(1).assert('got 1')

// Read the remaining entries:
const entries = log.read()

// Use a separate assertion library to do fancy tests:
expect(entries[0]).to.not.match(/Error/)

The log also has waitFor method, which is like an asynchronous version of read. It waits for the requested number of entries to arrive, and then returns them in a promise. The returned promise also has an assert method for convenience:

setTimeout(() => log('timeout 1'), 10)
setTimeout(() => log('timeout 2'), 15)
await log.waitFor(2).assert('timeout 1', 'timeout 2')

The waitFor method will stop waiting after 1 second and just return whatever is available. If you want to tweak this timeout, just pass a timeout option to makeAssertLog (in milliseconds):

const log = makeAssertLog({ timeout: 100 })

The makeAssertLog function also takes a verbose boolean option, which copies the log entries to console.log. This can be helpful while debugging, but isn't something you would normally leave turned on:

const log = makeAssertLog({ verbose: true })

Pretty-printing

The log function converts its arguments to strings using a function called stringify. You can access this function yourself, which can be useful if you don't know the exact log contents ahead of time:

import { makeAssertLog, stringify } from 'assert-log'

const log = makeAssertLog()

// Logs something like 'Got event { type: "grid", location: [0.1, 0.2] }':
const event = {
  type: 'grid',
  location: [Math.random(), Math.random()]
}
log('Got event', event)

// Use `stringify` to help build the expected message:
log.assert(`Got event ${stringify(event)}`)