[typescript] How can I declare a global variable in Angular 2 / Typescript?

I would like some variables to be accessible everywhere in an Angular 2 in the Typescript language. How should I go about accomplishing this?

That's the way I use it:

global.ts

export var server: string = 'http://localhost:4200/';
export var var2: number = 2;
export var var3: string = 'var3';

to use it just import like that:

import { Injectable } from '@angular/core';
import { Http, Headers, RequestOptions } from '@angular/http';
import { Observable } from 'rxjs/Rx';
import * as glob from '../shared/global'; //<== HERE

@Injectable()
export class AuthService {
    private AuhtorizationServer = glob.server
}

EDITED: Droped "_" prefixed as recommended.

I like the answer of @supercobra, but I would use the const keyword as it is in ES6 already available:

//
// ===== File globals.ts    
//
'use strict';

export const sep='/';
export const version: string="22.2.2"; 

I don't know the best way, but the easiest way if you want to define a global variable inside of a component is to use window variable to write like this:

window.GlobalVariable = "what ever!"

you don't need to pass it to bootstrap or import it other places, and it is globally accessibly to all JS (not only angular 2 components).

Create Globals class in app/globals.ts:

import { Injectable } from '@angular/core';

Injectable()
export class Globals{
    VAR1 = 'value1';
    VAR2 = 'value2';
}

In your component:

import { Globals } from './globals';

@Component({
    selector: 'my-app',
    providers: [ Globals ],
    template: `<h1>My Component {{globals.VAR1}}<h1/>`
})
export class AppComponent {
    constructor(private globals: Globals){
    }
}

Note: You can add Globals service provider directly to the module instead of the component, and you will not need to add as a provider to every component in that module.

@NgModule({
    imports: [...],
    declarations: [...],
    providers: [ Globals ],
    bootstrap: [ AppComponent ]
})
export class AppModule {
}

IMHO for Angular2 (v2.2.3) the best way is to add services that contain the global variable and inject them into components without the providers tag inside the @Component annotation. By this way you are able to share information between components.

A sample service that owns a global variable:

import { Injectable } from '@angular/core'

@Injectable()
export class SomeSharedService {
  public globalVar = '';
}

A sample component that updates the value of your global variable:

import { SomeSharedService } from '../services/index';

@Component({
  templateUrl: '...'
})
export class UpdatingComponent {

  constructor(private someSharedService: SomeSharedService) { }

  updateValue() {
    this.someSharedService.globalVar = 'updated value';
  }
}

A sample component that reads the value of your global variable:

import { SomeSharedService } from '../services/index';

@Component({
  templateUrl: '...'
})
export class ReadingComponent {

  constructor(private someSharedService: SomeSharedService) { }

  readValue() {
    let valueReadOut = this.someSharedService.globalVar;
    // do something with the value read out
  }
}

Note that providers: [ SomeSharedService ] should not be added to your @Component annotation. By not adding this line injection will always give you the same instance of SomeSharedService. If you add the line a freshly created instance is injected.

I like the solution from @supercobra too. I just would like to improve it slightly. If you export an object which contains all the constants, you could simply use es6 import the module without using require.

I also used Object.freeze to make the properties become true constants. If you are interested in the topic, you could read this post.

// global.ts

 export const GlobalVariable = Object.freeze({
     BASE_API_URL: 'http://example.com/',
     //... more of your variables
 });

Refer the module using import.

//anotherfile.ts that refers to global constants
import { GlobalVariable } from './path/global';

export class HeroService {
    private baseApiUrl = GlobalVariable.BASE_API_URL;

    //... more code
}

A shared service is the best approach

export class SharedService {
  globalVar:string;
}

But you need to be very careful when registering it to be able to share a single instance for whole your application. You need to define it when registering your application:

bootstrap(AppComponent, [SharedService]);

But not to define it again within the providers attributes of your components:

@Component({
  (...)
  providers: [ SharedService ], // No
  (...)
})

Otherwise a new instance of your service will be created for the component and its sub-components.

You can have a look at this question regarding how dependency injection and hierarchical injectors work in Angular 2:

• What's the best way to inject one service into another in angular 2 (Beta)?

You should notice that you can also define Observable properties in the service to notify parts of your application when your global properties change:

export class SharedService {
  globalVar:string;
  globalVarUpdate:Observable<string>;
  globalVarObserver:Observer;

  constructor() {
    this.globalVarUpdate = Observable.create((observer:Observer) => {
      this.globalVarObserver = observer;
    });
  }

  updateGlobalVar(newValue:string) {
    this.globalVar = newValue;
    this.globalVarObserver.next(this.globalVar);
  }
}

See this question for more details:

• Delegation: EventEmitter or Observable in Angular

I think the best way is to share an object with global variables throughout your application by exporting and importing it where you want.

First create a new .ts file for example globals.ts and declare an object. I gave it an Object type but you also could use an any type or {}

export let globalVariables: Object = {
 version: '1.3.3.7',
 author: '0x1ad2',
 everything: 42
};

After that import it

import {globalVariables} from "path/to/your/globals.ts"

And use it

console.log(globalVariables);

See for example Angular 2 - Implementation of shared services

@Injectable() 
export class MyGlobals {
  readonly myConfigValue:string = 'abc';
}

@NgModule({
  providers: [MyGlobals],
  ...
})

class MyComponent {
  constructor(private myGlobals:MyGlobals) {
    console.log(myGlobals.myConfigValue);
  }
}

or provide individual values

@NgModule({
  providers: [{provide: 'myConfigValue', useValue: 'abc'}],
  ...
})

class MyComponent {
  constructor(@Inject('myConfigValue') private myConfigValue:string) {
    console.log(myConfigValue);
  }
}