feat: add generic type to json method

[rtritto]

Features

Add support of generic type for json method of fetch and request.

Example

import { request } from 'undici'
const response = await request('URL')
const json = await response.body.json<{ data: 'myDataType' }>()

// ---

import { fetch } from 'undici'
const response = await fetch('URL')
const json = await response.json<{ data: 'myDataType' }>()

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

[Copilot]

Copilot wasn't able to review any files in this pull request.

---

💡 Add Copilot custom instructions for smarter, more guided reviews. Learn how to get started.

[Zamralik]

The json method do not provide a mean to validate the resulting value and it cannot guarantee what the received data are.
It is equivalent to using as, but passing the burden of "lying" about the type to the json method.

const json = await response.body.json() as { data: 'myDataType' };

Worse, it would implicitly cause the type to be erroneously "lied" about when used this way:

const json: { data: 'myDataType' } = await response.body.json();

As typescript implicitly fills the generic to .json<{ data: 'myDataType' }()

It may lead to implicit bugs and vulnerabilities from assuming the incoming data match the expected type.

[KhafraDev]

similar PRs have been rejected, you can find the reason we declined this in those PRs