Testing code with promises

[karpikpl]

Hey, I'm trying to test code that uses promises and always getting the MemoryLeakError. Is there anything I'm doing wrong, or is that just how promises work?

here's my code:

const Leakage =  require('leakage');

it('leak test', async() => {
    let i = 0;
    jasmine.DEFAULT_TIMEOUT_INTERVAL = 100000;
    expect.hasAssertions();

     await Leakage.iterate.async(async () => {
        let dummyPromise = new Promise((resolve) => {
           setTimeout(() => resolve('ok ' + ++i), 1);
        });

        try {
            const result = await dummyPromise;
            expect(result).toBeTruthy();
        } catch (e) {
            throw 'this should not happen ' + e;
        }
    });
})

and very minimalistic package.json

{
  "name": "promise-leak",
  "version": "1.0.0",
  "main": "index.js",
  "scripts": {
      "test": "jest --runInBand"
  },
  "license": "MIT",
  "dependencies": {
    "jest": "^23.6.0",
    "leakage": "^0.4.0"
  }
}

[andywer]

Hey @karpikpl!

Your code looks fine so far. Haven't ever tested the lib with Jasmine, though.

Can you please post the exact error output and ideally a heap dump (maybe in a GitHub gist or so)?

[karpikpl]

So I have good news and bad news. Good news is that the test is passing with mocha but failing with jest.

Same test, just different testing framework:

const Leakage =  require('leakage');
var assert = require('assert');

describe('leaks', function() {

    let expected, actual;

    beforeEach(function() {
        expected = 0;
        actual = 0;
    });

    afterEach(function() {
        if (!expected || expected == actual) return;
          var err = new Error('expected ' + expected + ' assertions, got ' + actual);
          this.currentTest.emit('error', err);
    });

    it('leak test', function() {
        this.timeout(100000);
        expected = 181;

         return Leakage.iterate.async(async () => {
            let dummyPromise = new Promise((resolve) => {
               setTimeout(() => resolve('ok'), 1);
            });

            try {
                const result = await dummyPromise;
                assert.equal(result, 'ok');
                actual++;
            } catch (e) {
                throw 'this should not happen ' + e;
            }
        });
    })
});

With mocha it passes and it's also much faster - 5s.

With Jest it fails but it's also much slower

$ jest --runInBand
 FAIL  tests/test.js (26.959s)
  ● leak test

    MemoryLeakError: Heap grew on 6 subsequent garbage collections (180 of 180 iterations) by 362 kB.

      Iterations between GCs: 30

      Final GC details:
      [   30.3 kB] [+ 378x] [-   2x] system / Context
      [   23.4 kB] [+ 378x] [-   1x] Closure
      [   21.9 kB] [+ 103x] [-   4x] Array
      [    3.6 kB] [+  31x] [-   1x] Timeout
      ... (9 more)

      at testConstantHeapSize (node_modules/leakage/lib/testConstantHeapSize.js:24:12)
      at Object.onAllDone (node_modules/leakage/lib/index.js:130:27)
      at Immediate.onHeapDiff (node_modules/leakage/lib/index.js:114:16)

I was unable to pass the --heap-file heap-diff.json param to jest. It worked with mocha, but mocha doesn't have the issue

[andywer]

Thanks for the comprehensive update! 👍🙂

I suppose we can turn that into one actionable only: Document that you should use tape or mocha; test runners that come with a minimal overhead and don‘t do much magic.