A barrel is a way to rollup exports from several modules into a single convenient module. The barrel itself is a module file that re-exports selected exports of other modules.
Imagine the following class structure in a library:
// demo/foo.ts
export class Foo {}
// demo/bar.ts
export class Bar {}
// demo/baz.ts
export class Baz {}
Without a barrel, a consumer would need three import statements:
import { Foo } from '../demo/foo';
import { Bar } from '../demo/bar';
import { Baz } from '../demo/baz';
You can instead add a barrel demo/index.ts containing the following:
// demo/index.ts
export * from './foo'; // re-export all of its exports
export * from './bar'; // re-export all of its exports
export * from './baz'; // re-export all of its exports
Now the consumer can import what it needs from the barrel:
import { Foo, Bar, Baz } from '../demo'; // demo/index.ts is implied
Named exports
Instead of exporting *, you can choose to export the module in a name. E.g., assume that baz.ts has functions:
// demo/foo.ts
export class Foo {}
// demo/bar.ts
export class Bar {}
// demo/baz.ts
export function getBaz() {}
export function setBaz() {}
If you would rather not export getBaz / setBaz from demo you can instead put them in a variable by importing them in a name and exporting that name as shown below:
// demo/index.ts
export * from './foo'; // re-export all of its exports
export * from './bar'; // re-export all of its exports
import * as baz from './baz'; // import as a name
export { baz }; // export the name
And now the consumer would look like:
import { Foo, Bar, baz } from '../demo'; // demo/index.ts is implied
// usage
baz.getBaz();
baz.setBaz();
// etc. ...
