What are all the index.ts used for?

I've been looking at a few seed projects and all the components seem to have a index.ts that exports * from that component. I can't find anywhere what it's actually used for?

E.g

Thanks

3 Answers

From the Angular.io v2's archived glossary entry for Barrel^*:

A barrel is a way to rollup exports from several modules into a single convenience module. The barrel itself is a module file that re-exports selected exports of other modules.

Imagine three modules in a heroes folder:

// heroes/hero.component.ts export class HeroComponent {} // heroes/hero.model.ts export class Hero {} // heroes/hero.service.ts export class HeroService {} 

Without a barrel, a consumer would need three import statements:

import { HeroComponent } from '../heroes/hero.component.ts'; import { Hero } from '../heroes/hero.model.ts'; import { HeroService } from '../heroes/hero.service.ts'; 

We can add a barrel to the heroes folder (called index by convention) that exports all of these items:

export * from './hero.model.ts'; // re-export all of its exports export * from './hero.service.ts'; // re-export all of its exports export { HeroComponent } from './hero.component.ts'; // re-export the named thing 

Now a consumer can import what it needs from the barrel.

import { Hero, HeroService } from '../heroes'; // index is implied 

The Angular scoped packages each have a barrel named index.

See also EXCEPTION: Can't resolve all parameters

^* NOTE: Barrel has been removed from more recent versions of the Angular glossary.

UPDATE With latest versions of Angular, barrel file should be edited as below,

export { HeroModel } from './hero.model'; export { HeroService } from './hero.service'; export { HeroComponent } from './hero.component'; 

index.ts is similar index.js in nodejs or index.html is web site hosting.

So when you say import {} from 'directory_name' it will look for index.ts inside the specified directory and import whatever is exported there.

For example if you have calculator/index.ts as

export function add() {...} export function multiply() {...} 

You can do

import { add, multiply } from './calculator'; 

index.ts help us to keep all related thing together and we don't need to worry about the source file name.

We can import all thing by using source folder name.

import { getName, getAnyThing } from './util'; 

Here util is folder name not file name which has index.ts which re-export all four files.

export * from './util1'; export * from './util2'; export * from './util3'; export * from './util4';