Migrate to named exports

Background

The JavaScript and TypeScript style guides specifically prohibit default exports for ES/TS Modules:

// Do not use default exports:
export default class Foo { ... } // BAD!

// Use named exports:
export class Foo { ... }

// Alternate style named exports:
class Foo { ... }
export {Foo};

For goog.module modules, the JavaScript style guide does not specifically prohibit default exports (exports = MyClass;), but it gives only examples of named exports. (It does give examples of goog.requireing the default export, however.)

Proposal

Migrate all modules which have a default export to use only named exports, so

const MyClass = function(...) {...};
exports = MyClass

becomes

const MyClass = function(...) {...};
exports.MyClass = MyClass;

and the corresponding import is (simultaneously) changed from

const MyClass = goog.require('MyClass');

to

const {MyClass} = goog.require('MyClass');

Discussion

There’s nothing wrong with default exports in the short term, but this is something we will want to fix eventually as we continue the migration from goog.provide -> goog.module -> ES modules -> TypeScript. Issue #5073 records a number of cases where we will probably decide to fix this sooner rather than later. This bug is to ensure that “later” arrives eventually.

[moniika]

But I believe this catch all the cases where we do non-destructuring imports, even the ones where there wasn’t a default export:

# Change all the default imports to named imports
grep -rlE "^const \w+ = goog\.require" core | xargs sed -i -E "s/^const (\w+) = goog\.require/const \{\1\} = goog\.require/"

I see. Good catch! I think we could still write a short temporary script to do this, but we would need something a tad more complex. Something that would save matches of module names that match the first step and use those in the second step. We’d need to save the

[moniika]

When the time comes, we can automate changing all this with the following commands (swapping out core for each directory we want to fix):

# Change all the default exports to named exports
grep -rlE "^exports = \w+" core | xargs sed -i -E "s/^exports = (\w+)/exports = \{\1\}/"
# Change all the default imports to named imports
grep -rlE "^const \w+ = goog\.require" core | xargs sed -i -E "s/^const (\w+) = goog\.require/const \{\1\} = goog\.require/"