What is the difference between declarations, providers, and import in NgModule?

Question

I am trying to understand Angular (sometimes called Angular2+), then I came across @Module:

1. Imports

2. Declarations

3. Providers

Following Angular Quick Start

Answer 1

imports are used to import supporting modules like FormsModule, RouterModule, CommonModule, or any other custom-made feature module.

declarations are used to declare components, directives, pipes that belong to the current module. Everyone inside declarations knows each other. For example, if we have a component, say UsernameComponent, which displays a list of the usernames and we also have a pipe, say toupperPipe, which transforms a string to an uppercase letter string. Now If we want to show usernames in uppercase letters in our UsernameComponent then we can use the toupperPipe which we had created before but the question is how UsernameComponent knows that the toupperPipe exists and how it can access and use that. Here come the declarations, we can declare UsernameComponent and toupperPipe.

Providers are used for injecting the services required by components, directives, pipes in the module.

Answer 2

Angular Concepts

• imports makes the exported declarations of other modules available in the current module
• declarations are to make directives (including components and pipes) from the current module available to other directives in the current module. Selectors of directives, components or pipes are only matched against the HTML if they are declared or imported.
• providers are to make services and values known to DI (dependency injection). They are added to the root scope and they are injected to other services or directives that have them as dependency.

A special case for providers are lazy loaded modules that get their own child injector. providers of a lazy loaded module are only provided to this lazy loaded module by default (not the whole application as it is with other modules).

For more details about modules see also https://angular.io/docs/ts/latest/guide/ngmodule.html

• exports makes the components, directives, and pipes available in modules that add this module to imports. exports can also be used to re-export modules such as CommonModule and FormsModule, which is often done in shared modules.

• entryComponents registers components for offline compilation so that they can be used with ViewContainerRef.createComponent(). Components used in router configurations are added implicitly.

TypeScript (ES2015) imports

import ... from 'foo/bar' (which may resolve to an index.ts) are for TypeScript imports. You need these whenever you use an identifier in a typescript file that is declared in another typescript file.

Angular's @NgModule() imports and TypeScript import are entirely different concepts.

See also jDriven - TypeScript and ES6 import syntax

Most of them are actually plain ECMAScript 2015 (ES6) module syntax that TypeScript uses as well.