Merge context option between instances

[ghermeto]

PR makes the context option enumerable and performs a shallow merge between instances.

It enables the following use-case:

test('extending context', withServer, async (t, server) => {
	server.get('/', (req, res) => {
		res.json(req.headers);
	});

	const pluginA = got.extend({
		prefixUrl: server.url,
		responseType: 'json',
		context: { test: true },
		hooks: {
			beforeRequest: [
				function injectClientInfo(options) {
					t.is(options.context.test, true);
				}
			]
		}
	});

	const pluginB = got.extend({
		headers: { 'x-test': 'test' },
		context: { something: 'else' }
	});

	const userInstance = got.extend(pluginA, pluginB);
	await userInstance.get('', { context: {} });
});

Closes #1443

Checklist

• I have read the documentation.
• I have included a pull request description of my changes.
• I have included some tests.
• If it's a new feature, I have included documentation updates in both the README and the types.

[szmarczak]

It's worth mentioning that it is enumerable and if it has defined non-enumerable options they will NOT be merged.

Avoid

const context = {};

Object.defineProperty(context, 'secret', {
	value: 'foobar',
	enumerable: false
});

Recommended

const context = {
	mySecrets: {}
};

Object.defineProperty(context.mySecrets, 'secret', {
	value: 'foobar',
	enumerable: false
});

/cc @sindresorhus

[szmarczak]

This is potentially breaking but we can try to publish this as a minor version and if there are any issues we can just revert this and push to Got 12.

[sindresorhus]

This is potentially breaking but we can try to publish this as a minor version and if there are any issues we can just revert this and push to Got 12.

That seems risky. It might change behavior in subtle ways. I would prefer to put this in Got 12. We could start preparing for Got 12. It's been a while since we did a major release. We do have some things queued up: https://github.com/sindresorhus/got/milestone/9

[ghermeto]

great! That works for me. I will update the test on this PR

[szmarczak]

(added #1443 to the milestone instead)

[sindresorhus]

@ghermeto Friendly bump :)

[ghermeto]

sorry, drop the ball on this one 😞. Will update it tonight.

[ghermeto]

seems like all Nodev12 tests have one timeout failure, which is unrelated to my fork as other PRs have recently failed on that particular test: https://travis-ci.com/github/sindresorhus/got/builds/191375693