default export in gatsby-ssr/browser doesn't produce warnings

[pieh]

Right now we produce error when there are named export in gatsby-ssr or gatsby-browser that don't match any of our API hooks ( https://www.gatsbyjs.org/docs/ssr-apis/ / https://www.gatsbyjs.org/docs/browser-apis/)

For example:

// in gatsby-ssr.js
export const notExistingAPIHook = () => {
  console.log("this won't show")
}

will produce

 ERROR #11329 

Your plugins must export known APIs from their gatsby-ssr.js.

See https://www.gatsbyjs.org/docs/ssr-apis/ for the list of Gatsby ssr APIs.

- Your local gatsby-ssr.js is using the API "notExistingAPIHook" which is not a known API.

When there is default export:

// in gatsby-ssr.js
export default () => {
  console.log("this won't show")
}

We don't get any warnings or errors, but it won't ever be executed by gatsby. Behaviour for exports should be consistent and we should warn about default exports as well

Task

Adjust

gatsby/packages/gatsby/src/bootstrap/resolve-module-exports.js

Lines 50 to 98 in d2fa8f7

	// extract names of exports from file
	traverse(ast, {
	// Check if the file is using ES6 imports
	ImportDeclaration: function ImportDeclaration(astPath) {
	isES6 = true
	},
	// get foo from `export const foo = bar`
	ExportNamedDeclaration: function ExportNamedDeclaration(astPath) {
	const exportName = get(
	astPath,
	`node.declaration.declarations[0].id.name`
	)
	isES6 = true
	if (exportName) exportNames.push(exportName)
	},
	// get foo from `export { foo } from 'bar'`
	// get foo from `export { foo }`
	ExportSpecifier: function ExportSpecifier(astPath) {
	const exportName = get(astPath, `node.exported.name`)
	isES6 = true
	if (exportName) exportNames.push(exportName)
	},
	AssignmentExpression: function AssignmentExpression(astPath) {
	const nodeLeft = astPath.node.left
	if (nodeLeft.type !== `MemberExpression`) return
	// ignore marker property `__esModule`
	if (get(nodeLeft, `property.name`) === `__esModule`) return
	// get foo from `exports.foo = bar`
	if (get(nodeLeft, `object.name`) === `exports`) {
	isCommonJS = true
	exportNames.push(nodeLeft.property.name)
	}
	// get foo from `module.exports.foo = bar`
	if (
	get(nodeLeft, `object.object.name`) === `module` &&
	get(nodeLeft, `object.property.name`) === `exports`
	) {
	isCommonJS = true
	exportNames.push(nodeLeft.property.name)
	}
	},
	})

to handle default exports as well and ideally add test case for that in https://github.com/gatsbyjs/gatsby/blob/master/packages/gatsby/src/bootstrap/__tests__/resolve-module-exports.js

[danilobuerger]

Related to this, when keeping the ssr code somewhere else, including it like:

export * from "./src/utils/gatsby-ssr";

fails silently too.