Define global constants

Question

In Angular 1 x you can define constants like this   angular module  mainApp config            constant  API ENDPOINT    http   127 0 0 1 6666 api      What would be the equivalent in Angular  with TypeScript    I just don t want to repeat the API base url over and over again in all my services

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In Angular2  you have the following provide  definition  which allows you to setup different kinds of dependencies   provide token  any   useClass  useValue  useExisting  useFactory  deps  multi    Comparing to Angular 1  app service in Angular1 is equivalent to useClass in Angular2   app factory in Angular1 is equivalent to useFactory in Angular2   app constant and app value has been simplified to useValue with less constraints  i e  there is no config block anymore   app provider - There is no equivalent in Angular 2   Examples  To setup with the root injector   bootstrap AppComponent  provide API ENDPOINT    useValue  http   127 0 0 1 6666 api           Or setup with your component s injector   providers   provide API ENDPOINT    useValue   http   127 0 0 1 6666 api        provide is short hand for   var injectorValue   Injector resolveAndCreate     new Provider API ENDPOINT    useValue   http   127 0 0 1 6666 api           With the injector  getting the value is easy   var endpoint   injectorValue get API ENDPOINT

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This is my recent experience with this scenario     angular cli  1 0 0 node  6 10 2  angular core  4 0 0   I ve followed the official and updated docs here   https   angular io docs ts latest guide dependency-injection html   dependency-injection-tokens  Seems OpaqueToken is now deprecated and we must use InjectionToken  so these are my files running like a charm   app-config interface ts  export interface IAppConfig      STORE KEY  string       app-config constants ts  import   InjectionToken   from   angular core   import   IAppConfig   from    app-config interface    export const APP DI CONFIG  IAppConfig        STORE KEY   l  list        export let APP CONFIG   new InjectionToken lt  IAppConfig  gt    app config       app module ts  import   APP CONFIG  APP DI CONFIG   from    app-config app-config constants     NgModule      declarations             imports             providers                         provide  APP CONFIG        useValue  APP DI CONFIG              bootstrap              export class AppModule      my-service service ts    constructor                       Inject  APP CONFIG   private config  IAppConfig         console log  This is the App s Key     this config STORE KEY          gt  This is the App s Key   l  list         The result is clean and there s no warnings on console thank s to recent comment of John Papa in this issue   https   github com angular angular-cli issues 2034  The key was implement in a different file the interface

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The solution for the configuration provided by the angular team itself can be found here   Here is all the relevant code   1  app config ts  import   OpaqueToken   from   angular core    export let APP CONFIG   new OpaqueToken  app config     export interface IAppConfig       apiEndpoint  string     export const AppConfig  IAppConfig             apiEndpoint   http   localhost 15422 api            2  app module ts  import   APP CONFIG  AppConfig   from    app config     NgModule       providers              provide  APP CONFIG  useValue  AppConfig              3  your service ts  import   APP CONFIG  IAppConfig   from    app config     Injectable   export class YourService        constructor  Inject APP CONFIG  private config  IAppConfig                    You can use config apiEndpoint now              Now you can inject the config everywhere without using the string names and with the use of your interface for static checks   You can of course separate the Interface and the constant further to be able to supply different values in production and development e g

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One approach for Angular4 would be defining a constant at module level   const api endpoint    http   127 0 0 1 6666 api      NgModule     declarations   AppComponent     bootstrap   AppComponent     providers        MessageService       provide   API ENDPOINT   useValue  api endpoint         export class AppModule       Then  in your service   import  Injectable  Inject  from   angular core     Injectable   export class MessageService        constructor private http  Http          Inject  API ENDPOINT   private api endpoint  string           getMessages    Observable lt Message   gt            return this http get this api endpoint   messages                map response   gt  response json                 map  messages  Object      gt                    return messages map message   gt  this parseData message                               private parseData data   Message           return new Message data

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The best way to create application wide constants in Angular 2 is by using environment ts files  The advantage of declaring such constants is that you can vary them according to the environment as there can be a different environment file for each environment

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Updated for Angular 4   Now we can simply use environments file which angular provide default if your project is generated via angular-cli   for example   In your environments folder create following files    environment prod ts environment qa ts environment dev ts   and each file can hold related code changes such as    environment prod ts  export const environment          production  true       apiHost   https   api somedomain com prod v1         CONSUMER KEY   someReallyStupidTextWhichWeHumansCantRead         codes     AB    AC    XYZ         environment qa ts  export const environment          production  false       apiHost   https   api somedomain com qa v1         CONSUMER KEY    someReallyStupidTextWhichWeHumansCantRead         codes     AB    AC    XYZ         environment dev ts  export const environment          production  false       apiHost   https   api somedomain com dev v1         CONSUMER KEY    someReallyStupidTextWhichWeHumansCantRead         codes     AB    AC    XYZ           Use-case in application  You can import environments into any file such as services  clientUtilServices ts   import  environment  from        environments environment    getHostURL    string       return environment apiHost        Use-case in build  Open your angular cli file  angular-cli json and inside  apps           add following code     apps              environments                  dev    environments environment ts                prod    environments environment prod ts                qa    environments environment qa ts                                      If you want to build for production  run  ng build --env prod it will read configuration from  environment prod ts   same way you can do it for qa or dev     Older answer  I have been doing something like below  in my provider   import  Injectable  from   angular core     Injectable   export class ConstantService    API ENDPOINT  String  CONSUMER KEY   String   constructor         this API ENDPOINT    https   api somedomain com v1        this CONSUMER KEY    someReallyStupidTextWhichWeHumansCantRead          Then i have access to all Constant data at anywhere   import  Injectable  from   angular core   import  Http  from   angular http   import  rxjs add operator map    import  ConstantService  from     constant-service     This is my Constant Service    Injectable   export class ImagesService       constructor public http  Http  public ConstantService  ConstantService        console log  Hello ImagesService Provider            callSomeService          console log  API ENDPOINT    this ConstantService API ENDPOINT       console log  CONSUMER KEY    this ConstantService CONSUMER KEY       var url   this ConstantService API ENDPOINT      return this http get url

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Below changes works for me on Angular 2 final version   export class AppSettings      public static API ENDPOINT  http   127 0 0 1 6666 api        And then in the service   import  Http  from  angular2 http   import  Message  from     models message   import  Injectable  from  angular2 core   import  Observable  from  rxjs Observable   import  AppSettings  from     appSettings   import  rxjs add operator map     Injectable   export class MessageService        constructor private http  Http           getMessages    Observable lt Message   gt            return this http get AppSettings API ENDPOINT   messages                map response   gt  response json                 map  messages  Object      gt                    return messages map message   gt  this parseData message                               private parseData data   Message           return new Message data

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Just use a Typescript constant  export var API ENDPOINT    http   127 0 0 1 6666 api      You can use it in the dependency injector using  bootstrap AppComponent   provide API ENDPOINT   useValue   http   127 0 0 1 6666 api

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In Angular 4  you can use environment class to keep all your globals   You have environment ts and environment prod ts by default   For example  export const environment       production  false    apiUrl   http   localhost 8000 api        And then on your service   import   environment   from        environments environment       environment apiUrl

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AngularJS s module constant does not define a constant in the standard sense   While it stands on its own as a provider registration mechanism  it is best understood in the context of the related module value   provide value  function  The official documentation states the use case clearly      Register a value service with the  injector  such as a string  a number  an array  an object or a function  This is short for registering a service where its provider s  get property is a factory function that takes no arguments and returns the value service  That also means it is not possible to inject other services into a value service    Compare this to the documentation for module constant   provide constant  which also clearly states the use case  emphasis mine       Register a constant service with the  injector  such as a string  a number  an array  an object or a function  Like the value  it is not possible to inject other services into a constant    But unlike value  a constant can be injected into a module configuration function  see angular Module  and it cannot be overridden by an AngularJS decorator    Therefore  the AngularJS constant function does not provide a constant in the commonly understood meaning of the term in the field   That said the restrictions placed on the provided object  together with its earlier availability via the  injector  clearly suggests that the name is used by analogy   If you wanted an actual constant in an AngularJS application  you would  provide  one the same way you would in any JavaScript program which is  export const p   3 14159265    In Angular 2  the same technique is applicable   Angular 2 applications do not have a configuration phase in the same sense as AngularJS applications  Furthermore  there is no service decorator mechanism  AngularJS Decorator  but this is not particularly surprising given how different they are from each other   The example of  angular    module  mainApp config          constant  API ENDPOINT    http   127 0 0 1 6666 api       is vaguely arbitrary and slightly off-putting because  provide constant is being used to specify an object that is incidentally also a constant  You might as well have written  export const apiEndpoint    http   127 0 0 1 6666 api      for all either can change   Now the argument for testability  mocking the constant  is diminished because it literally does not change   One does not mock p   Of course your application specific semantics might be that your endpoint could change  or your API might have a non-transparent failover mechanism  so it would be reasonable for the API endpoint to change under certain circumstances   But in that case  providing it as a string literal representation of a single URL to the constant function would not have worked   A better argument  and likely one more aligned with the reason for the existence of the AngularJS  provide constant function is that  when AngularJS was introduced  JavaScript had no standard module concept  In that case  globals would be used to share values  mutable or immutable  and using globals is problematic   That said  providing something like this through a framework increases coupling to that framework  It also mixes Angular specific logic with logic that would work in any other system   This is not to say it is a wrong or harmful approach  but personally  if I want a constant in an Angular 2 application  I will write  export const p   3 14159265    just as I would have were I using AngularJS   The more things change

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If you re using Webpack  which I recommend  you can set-up constants for different environments  This is especially valuable when you have different constant values on a per environment basis   You ll likely have multiple webpack files under your  config directory  e g   webpack dev js  webpack prod js  etc     Then you ll have a custom-typings d ts you ll add them there  Here s the general pattern to follow in each file and a sample usage in a Component   webpack  env  js  const API URL   process env API URL    http   localhost 3000    const JWT TOKEN NAME    id token           plugins             NOTE  when adding more properties  make sure you include them in custom-typings d ts       new DefinePlugin            API URL   JSON stringify API URL            JWT TOKEN NAME   JSON stringify JWT TOKEN NAME              custom-typings d ts  declare var API URL  string  declare var JWT TOKEN NAME  string  interface GlobalEnvironment     API URL  string    JWT TOKEN NAME  string      Component  export class HomeComponent implements OnInit     api url string   API URL    authToken  string    Bearer     localStorage getItem JWT TOKEN NAME

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While the approach with having a AppSettings class with a string constant as ApiEndpoint works  it is not ideal since we wouldn t be able to swap this real ApiEndpoint for some other values at the time of unit testing    We need to be able to inject this api endpoints into our services  think of injecting a service into another service   We also do not need to create a whole class for this  all we want to do is to inject a string into our services being our ApiEndpoint  To complete the excellent answer by pixelbits  here is the complete code as to how it can be done in Angular 2   First we need to tell Angular how to provide an instance of our ApiEndpoint when we ask for it in our app  think of it as registering a dependency        bootstrap AppComponent            HTTP PROVIDERS          provide  ApiEndpoint    useValue   http   127 0 0 1 6666 api                     And then in the service we inject this ApiEndpoint into the service constructor and Angular will provide it for us based on our registration above   import  Http  from  angular2 http   import  Message  from     models message   import  Injectable  Inject  from  angular2 core         We import Inject here import  Observable  from  rxjs Observable   import  AppSettings  from     appSettings   import  rxjs add operator map     Injectable   export class MessageService        constructor private http  Http                    Inject  ApiEndpoint   private apiEndpoint  string           getMessages    Observable lt Message   gt            return this http get    this apiEndpoint  messages                map response   gt  response json                 map  messages  Object      gt                    return messages map message   gt  this parseData message                                  the rest of the code

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I have another way to define global constants  Because if we defined in ts file  if build in production mode it is not easy to find constants to change value   export class SettingService       constructor private http  HttpClient            public getJSON file   Observable lt any gt          return this http get    assets configs     file     json          public getSetting             use setting here         In app folder  i add folder configs setting json  Content in setting json         baseUrl    http   localhost 52555      In app module add APP INITIALIZER             provide  APP INITIALIZER        useFactory   setting  SettingService    gt  function    return setting getSetting           deps   SettingService         multi  true         with this way  I can change value in json file easier  I also use this way for constant error warning messages

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Using a property file that is generated during a build is simple and easy  This is the approach that the Angular CLI uses  Define a property file for each environment and use a command during build to determine which file gets copied to your app  Then simply import the property file to use   https   github com angular angular-cli build-targets-and-environment-files

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You can make a class for your global variable and then export this class like this   export class CONSTANT       public static message2                NAME REQUIRED    Name is required               public static message              NAME REQUIRED    Name is required             After creating and exporting your CONSTANT class  you should import this class in that class where you want to use  like this   import   Component  OnInit                         from   angular core   import   CONSTANT                                  from        constants dash-constant      Component     selector      team-component     templateUrl      app modules dashboard dashComponents teamComponents team component html       export class TeamComponent implements OnInit     constructor         console log CONSTANT message2 0  NAME REQUIRED       console log CONSTANT message NAME REQUIRED          ngOnInit         console log  oninit        console log CONSTANT message2 0  NAME REQUIRED       console log CONSTANT message NAME REQUIRED           You can use this either in constructor or ngOnInit      or in any predefine methods

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All solutions seems to be complicated  I m looking for the simplest solution for this case and I just want to use constants  Constants are simple  Is there anything which speaks against the following solution   app const ts   use strict    export const dist       path to dist      app service ts  import   as AppConst from     app const      Injectable   export class AppService        constructor                   console log  dist path   AppConst dist

[typescript] Define global constants

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