memlab

memlab is an E2E testing and analysis framework for finding JavaScript memory
leaks and optimization opportunities.

Online Resources:
* Meta Engineering Blog Post
* Official Website and Demo
* Documentation

Features:
* Browser memory leak detection - Write test scenario with puppeteer API,
memlab auto diffs JS heap snapshots, filters out memory leaks, and
aggregates results.
* Object-oriented heap traversing API - Supports self-defined memory leak
detector and programmatically analyzing JS heap snapshots taken from
Chromium-based browsers, Node.js, Electron.js, and Hermes
* Memory CLI toolbox - Built-in toolbox and APIs for finding memory
optimization opportunities (not necessarily memory leaks)
* Memory assertions in Node.js - Enables unit test or running node.js
program to take a heap snapshot of its own state, do self memory checking,
or write advanced memory assertions

CLI Usage

Install the CLI

``bash
npm install -g memlab
`

$3

To find memory leaks in Google Maps, you can create a
scenario file defining how
to interact with the Google Maps, let's name it test-google-maps.js:

`javascript
// initial page load url: Google Maps
function url() {
return 'https://www.google.com/maps/@37.386427,-122.0428214,11z';
}

// action where we want to detect memory leaks: click the Hotels button
async function action(page) {
// puppeteer page API
await page.click('button[aria-label="Hotels"]');
}

// action where we want to go back to the step before: click clear search
async function back(page) {
// puppeteer page API
await page.click('[aria-label="Clear search"]');
}

module.exports = {action, back, url};
`

Now run memlab with the scenario file, memlab will interact with
the web page and detect memory leaks with built-in leak detectors:

`bash
memlab run --scenario test-google-maps.js
`

memlab will print memory leak results showing one representative
retainer trace for each cluster of leaked objects.

Retainer traces: This is the result from
an example website,
the retainer trace is an object reference chain from the GC root to a leaked
object. The trace shows why and how a leaked object is still kept alive in
memory. Breaking the reference chain means the leaked object will no longer
be reachable from the GC root, and therefore can be garbage collected.
By following the leak trace one step at a time, you will be able to find
a reference that should be set to null (but it wasn't due to a bug).

`bash
MemLab found 46 leak(s)
--Similar leaks in this run: 4--
--Retained size of leaked objects: 8.3MB--
[Window] (native) @35847 [8.3MB]
--20 (element)---> [InternalNode] (native) @130981728 [8.3MB]
--8 (element)---> [InternalNode] (native) @130980288 [8.3MB]
--1 (element)---> [EventListener] (native) @131009888 [8.3MB]
--1 (element)---> [V8EventListener] (native) @224808192 [8.3MB]
--1 (element)---> [eventHandler] (closure) @168079 [8.3MB]
--context (internal)---> [] (object) @181905 [8.3MB]
--bigArray (variable)---> [Array] (object) @182925 [8.3MB]
--elements (internal)---> [(object elements)] (array) @182929 [8.3MB]
...
`
To get readable trace, the web site under test needs to serve non-minified code (or at least minified code
with readable variables, function name, and property names on objects).

Alternatively, you can debug the leak by loading the heap snapshot taken by memlab (saved in $(memlab get-default-work-dir)/data/cur)
in Chrome DevTool and search for the leaked object ID (@182929).

Self-defined leak detector: If you want to use a self-defined leak detector, add a filterLeak callback
(doc)
in the scenario file. filterLeak will be called for every unreleased heap
object (node) allocated by the target interaction.

`javascript
function filterLeak(node, heap) {
// ... your leak detector logic
// return true to mark the node as a memory leak
};
`

heap is the graph representation of the final JavaScript heap snapshot.
For more details, view the
doc site.

$3

View which object keeps growing in size during interaction in the previous run:
`bash
memlab analyze unbound-object
`

Analyze pre-fetched V8/hermes .heapsnapshot files:

`bash
memlab analyze unbound-object --snapshot-dir
`

Use memlab analyze to view all built-in memory analyses.
For extension, view the doc site.

View retainer trace of a particular object:
`bash
memlab trace --node-id
`

Use memlab help to view all CLI commands.

APIs

Use the memlab npm package to start a E2E run in browser and detect memory leaks.

`javascript
const memlab = require('memlab');

const scenario = {
// initial page load url
url: () => 'https://www.google.com/maps/@37.386427,-122.0428214,11z',

// action where we want to detect memory leaks
action: async (page) => await page.click('button[aria-label="Hotels"]'),

// action where we want to go back to the step before
back: async (page) => await page.click('[aria-label="Clear search"]'),
}
memlab.run({scenario});
`

Memory Assertions

memlab makes it possible to enable a unit test or running node.js program
to take a heap snapshot of its own state, and write advanced memory assertions:

`typescript
// save as example.test.ts
import type {IHeapSnapshot, Nullable} from '@memlab-test/core';
import {config, takeNodeMinimalHeap} from '@memlab-test/core';

class TestObject {
public arr1 = [1, 2, 3];
public arr2 = ['1', '2', '3'];
}

test('memory test with heap assertion', async () => {
config.muteConsole = true; // no console output

let obj: Nullable = new TestObject();
// get a heap snapshot of the current program state
let heap: IHeapSnapshot = await takeNodeMinimalHeap();

// call some function that may add references to obj
rabbitHole(obj)

expect(heap.hasObjectWithClassName('TestObject')).toBe(true);
obj = null;

heap = await takeNodeMinimalHeap();
// if rabbitHole does not have any side effect that
// adds new references to obj, then obj can be GCed
expect(heap.hasObjectWithClassName('TestObject')).toBe(false);

}, 30000);
``

For other APIs check out the
API documentation.