Jazzer.js jest-runner can run in two modes: regression, and fuzzing. Jazzer.js auto-generates the directories for each mode and test. In regression mode for your tests in regression.test.fuzz.js three directories are generated based on the describe blocks and test names in the test directory:

├── regression.test.fuzz
│   └── ensure_previous_fuzzer_findings_are_not_reintroduced
│       ├── console.log
│       ├── dom-parser.html.fuzz.js
│       └── dom-parser.xml.fuzz.js

Files in these three directories will be used for their corresponding tests.

In fuzzing mode, the directory is in the main project directory .cifuzz-corpus, where you put some files already (the first two will be ignored, because they are not associated with any test):

├── .cifuzz-corpus
│   ├── crash-doctype-in-element.xml
│   ├── crash-proto-prefix.xml
│   └── regression.test.fuzz
│       └── ensure_previous_fuzzer_findings_are_not_reintroduced
│           ├── console.log
│           │   ├── crash-doctype-in-element.xml
│           │   └── crash-proto-prefix.xml
│           ├── dom-parser.html.fuzz.js
│           │   ├── crash-doctype-in-element.xml
│           │   └── crash-proto-prefix.xml
│           └── dom-parser.xml.fuzz.js
│               ├── crash-doctype-in-element.xml
│               └── crash-proto-prefix.xml

The fuzzing mode also uses inputs from the regression directories of for each test, however regression mode only uses inputs from the regression directory.

Also, the first input that the fuzzer tries is an empty input (does not contain any bytes). If you have no files in the regression directory of the corresponding test, this will be the only input that the fuzzer will try. That should explain your screenshot above.

@oetr thank you for the explanation.
Sadly I still don't understand why the files I added are not considered by the regression tests.

I would have expected that the two samples are going to be logged by the test using console.log.

regression mode only uses inputs from the regression directory.

Do you mean I should add a regression sub directory to each of those directories and move the files there?

(Is there any documentation/executable example that shows this?)

Do you mean I should add a regression sub directory to each of those directories and move the files there?

Jazzer.js automatically generates all necessary directories for both modes (regression, fuzzing) after you run your jest fuzz tests once.

The regression tests are always in some folder relative to where the test resides.
The regression tests for your console.log test in test/regression.test.fuzz.js should go into test/regression.test.fuzz.js/ensure_previous_fuzzer_findings_are_not_reintroduced/console.log/

(Is there any documentation/executable example that shows this?)

You can check out the example in https://github.com/CodeIntelligenceTesting/jazzer.js/blob/main/examples/jest_integration/integration.fuzz.js, where the regression tests are in the subfolder ./integration.fuzz.js.

It is somewhat cryptically mentioned in https://github.com/CodeIntelligenceTesting/jazzer.js/blob/main/docs/jest-integration.md#fuzzing-mode, but I will add a ticket to improve this part of our documentation.

Thanks for reporting this!

Worked like a charm.

Here is my "generic" solution for a folder that contains a single regression testsuite and multiple fuzz targets, which reflects the directory structure that jazzer.js generates and will yell at you for targets without any input files and verifies that each target it called the expected amount of times:

'use strict';

const { describe, expect, test, beforeAll } = require('@jest/globals');
const fs = require('fs');
const path = require('path');
const TARGETS = fs.readdirSync(__dirname).filter((file) => file.endsWith('.target.js'));

TARGETS.forEach((target) => {
	describe('', () => {
		beforeAll(() => {
			const testfiles = fs.readdirSync(path.join(__filename.replace(/\.js$/, ''), target));
			expect(testfiles.length).toBeGreaterThan(0);
		});
		const module = require(path.join(__dirname, target));
		test.fuzz(target, (data) => module.fuzz(data));
	});
});

(Update: I needed to drop the assertion on the calls made to the spy, it didn't work reliably.)
Improved docs will sure help other people trying the same