.default() sanitizer not working when chained with .optional()

[megargayu]

Describe the bug

I am trying to use the default() sanitizer to make my optional array field have a default value. However, when I chain the default() sanitizer with other checks, the default() sanitizer doesn't do anything. In order for it to work, I need to make another check which only has the default() sanitizer attached to it. Instead of body("...").isArray().optional().default([]), I have to split it up into body("...").isArray().optional() and body("...").default([]).

To Reproduce

1. Create a new npm project with the following dependencies/dev dependencies:

@types/express@4.17.12
express-validator@6.12.0
express@4.17.1
typescript@4.3.4

2. Create a new typescript file with the following code:

import express from "express";
import { body } from "express-validator";

const app = express();

app.use(express.json());

app.post(
  "/fails",
  body("children").isArray().optional().default([]),
  (req: express.Request) => {
    console.log(`/fails output: ${JSON.stringify(req.body, null, 4)}`);
  }
);

app.post(
  "/works",
  body("children").isArray().optional(),
  body("children").default([]),
  (req: express.Request) => {
    console.log(`/works output: ${JSON.stringify(req.body, null, 4)}`);
  }
);

app.listen(3000);

3. Run the server and send a POST request with an empty body to both /fails and /works.

Expected behavior

When the server is run and an empty POST request is sent to both /fails and /works, the following should show up in the console:

/fails output: {
    "children": []
}
/works output: {
    "children": []
}

Current behavior

When the server is run and an empty POST request is sent to both /fails and /works, the following currently happens:

/fails output: {}
/works output: {
    "children": []
}

Express-validator version:

• Version: 6.12.0

[megargayu]

Looks like I had to remove optional(). When I removed optional(), /fails worked as well as /works (gives the correct output). I am going to close the issue with this, but could someone explain why this happens? Why does doing .isArray().optional().default([]) not work but .isArray().default([]) work? Is this a bug? Can it at least be documented?

[fedeci]

Reopening this so that it helps me keep track of thing to do.

[fedeci]

Note for whoever wants to try looking into this: probably when data is matched it does not take fields marked with .optional() into account.

[imjordanxd]

Also curious regarding the behaviour of this. In my scenario, children should be optional. But it should be an array if provided. My solution was to use default values when destructuring.

query("children").isArray().optional(),
(req, res) => {
	const { children = [] } = matchedData(req);
	....
}