Asserting a range with descriptive error messages

[nicholaswmin]

Node.js Version

v22

NPM Version

10.8.2

Operating System

MacOS Sonoma

Subsystem

assert

Description

I work a lot with performance testing code which frequently leads in the unfortunate position where I need to assert ranges rather than exact values, i.e

is value between 1500 and 2000?

Now, to do that I use a pair of assert.ok like so:

const value = 1700

assert.ok(value >= 1500)
assert.ok(value <= 2000)

so far so good, could be better but that's clean and concise enough for my taste.

The problem is that setting a descriptive error message quickly becomes rather repetitive and convoluted.

I have to resort to something akin to this:

assert.ok(value >= 1500, `expected value >= 1500 , got: ${value}`)
assert.ok(value <= 2000, `expected value <= 2000 , got: ${value}`)

Apart from cooking up my own assertion functions, is there a better way to do this out-the-box that I'm missing?

[avivkeller]

CC @nodejs/assert

[gireeshpunathil]

looks reasonable to me!

[gireeshpunathil]

@nicholaswmin - pls feel free to raise a PR if you are willing to!

[avivkeller]

Although, doesn't the assert message include the failed assertion line, so wouldn't it already show the failure?

[nicholaswmin]

@redyetidev here's the output:

import test from 'node:test'

await test('is within range',  async t => {
  const value = 2700
  
  t.assert.ok(value >= 1500)
  t.assert.ok(value <= 2000)
})

✖ is within range (1.220792ms)
  AssertionError [ERR_ASSERTION]: false == true
      at assert.<computed> [as ok] (node:internal/test_runner/test:261:18)
      at TestContext.<anonymous> (file:///Users/nicholaswmin/Projects/fsm/scratch.js:7:12)
      at Test.runInAsyncScope (node:async_hooks:211:14)
      at Test.run (node:internal/test_runner/test:878:25)
      at Test.processPendingSubtests (node:internal/test_runner/test:590:18)
      at node:internal/test_runner/harness:252:20
      at node:internal/process/task_queues:151:7
      at AsyncResource.runInAsyncScope (node:async_hooks:211:14)
      at AsyncResource.runMicrotask (node:internal/process/task_queues:148:8) {
    generatedMessage: true,
    code: 'ERR_ASSERTION',
    actual: false,
    expected: true,
    operator: '=='
  }

My "complaint" is that it doesn't print value being asserted against, unless I specifically code it to so with that string interpolation.

Now granted, the above is impossible to do with the code I'm providing because that's a Boolean expression.
An ideal API might be something like:

  t.assert.ok(value <= 2000, value)
  t.assert.ok(value <= 2000, value)

AssertionError [ERR_ASSERTION]: Failed, value: 2700 was not ok

Hmm.. I'm a bit confused... Not sure why I opened this issue as what I'm proposing sounds like a silly API.

Doing this:

t.assert.ok(value <= 2000, value)

prints the value anyway.

I don't remember what I had in mind. Sorry.

[avivkeller]

assert.isGreaterThan (and vise versa) could work if you'd like to add it?

[nicholaswmin]

I'm personally against adding specific assertion checks to assert like that.

It'll eventually snowball into a myriad utilities. I miss the hyper-declarative syntax of Chai but I've also come to love the minimal, no-frills API surface of assert since I don't need to memorise a million method names.

... I don't remember what i had in mind when i opened this.

If I need range checks I can extend assert locally.

I think this needs to be closed ??

[nicholaswmin]

The closest thing I can figure out that would help without adding specific assertion checks is printing the expression itself on assert.ok().

An example:

import test from 'node:test'

await test('foo', async t => {
  const value = 2700
  
  t.assert.ok(value >= 1500)
  t.assert.ok(value <= 2000)
})

logs:

✖ is in range (1.220875ms)
  AssertionError [ERR_ASSERTION]: `value >= 1500`
      at assert.<computed> [as ok] (node:internal/test_runner/test:261:18)
      at TestContext.<anonymous> (file:///Users/nicholaswmin/Projects/fsm/scratch.js:7:12)
      at Test.runInAsyncScope (node:async_hooks:211:14)
      at Test.run (node:internal/test_runner/test:878:25)
      at Test.processPendingSubtests (node:internal/test_runner/test:590:18)
      at node:internal/test_runner/harness:252:20
      at node:internal/process/task_queues:151:7
      at AsyncResource.runInAsyncScope (node:async_hooks:211:14)
      at AsyncResource.runMicrotask (node:internal/process/task_queues:148:8) {
    generatedMessage: true,
    code: 'ERR_ASSERTION',
    actual: false,
    expected: true,
    operator: '=='
  }

which is more descriptive than the current:

AssertionError [ERR_ASSERTION]: false == true

Not sure if that's doable without doing esprima-like parsing though?

[avivkeller]

For your specific case, a custom error (E.G. throw new assert.AssertionError({actual: 1, expected:2, operator:">="})) can be thrown.

Not sure if that's doable without doing esprima-like parsing though?

In some cases, that is what happens:

import assert from 'node:assert';

assert.ok(25 > 50);

node:internal/modules/run_main:123
    triggerUncaughtException(
    ^

AssertionError [ERR_ASSERTION]: The expression evaluated to a falsy value:

  assert.ok(25 > 50)

    at file:///Users/avivkeller/Documents/tmp/repro.mjs:3:8
    at ModuleJob.run (node:internal/modules/esm/module_job:262:25)
    at async onImport.tracePromise.__proto__ (node:internal/modules/esm/loader:482:26)
    at async asyncRunEntryPointWithESMLoader (node:internal/modules/run_main:117:5) {
  generatedMessage: true,
  code: 'ERR_ASSERTION',
  actual: false,
  expected: true,
  operator: '=='
}

Node.js v22.8.0

[nicholaswmin]

In some cases, that is what happens:

Yes. That works! That's more than enough.

I can repro your working example with assert but not with t.assert

[avivkeller]

I'm aware. I'm opening an issue for it as we speak.