How to define Singleton in TypeScript

Question

What is the best and most convenient way to implement a Singleton pattern for a class in TypeScript   Both with and without lazy initialisation

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This is a typescript decorator checking if multiple instances were accidentaly created against a service class that is designed to be singleton  https   www npmjs com package singleton-checker

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Singleton classes in TypeScript are generally an anti-pattern  You can simply use namespaces instead   Useless singleton pattern  class Singleton              lots of singleton logic            public someMethod                 Using var x   Singleton getInstance    x someMethod      Namespace equivalent  export namespace Singleton       export function someMethod                Usage import   SingletonInstance   from  path to Singleton    SingletonInstance someMethod    var x   SingletonInstance     If you need to alias it for some reason

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namespace MySingleton     interface IMySingleton         doSomething    void        class MySingleton implements IMySingleton         private usePrivate             doSomething               this usePrivate                  export var Instance  IMySingleton   new MySingleton        This way we can apply an interface  unlike in Ryan Cavanaugh s accepted answer

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My solution for it   export default class Modal       private static  instance   Modal   new Modal         constructor              if  Modal  instance               throw new Error  Use Modal instance            Modal  instance   this             static get instance              return Modal  instance

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This is probably the longest process to make a singleton in typescript  but in larger applications is the one that has worked better for me   First you need a Singleton class in  let s say     utils Singleton ts    module utils       export class Singleton           private  initialized  boolean           private  setSingleton    void               if  this  initialized  throw Error  Singleton is already initialized                 this  initialized   true                     get setSingleton     return this  setSingleton              Now imagine you need a Router singleton    navigation Router ts         lt reference path     utils Singleton ts    gt   module navigation       class RouterClass extends utils Singleton              NOTICE RouterClass extends from utils Singleton            and that it isn t exportable           private  init    void                  This method will be your  construtor  now                 to avoid double initialization  don t forget                the parent class setSingleton method               this setSingleton                    Initialization stuff                        Expose  init method          get init   return this init                  THIS IS IT   Export a new RouterClass  that no        one can instantiate ever again       export var Router  RouterClass   new RouterClass        Nice   now initialize or import wherever you need        lt reference path    navigation Router ts    gt   import router   navigation Router   router init    router init       Throws error     The nice thing about doing singletons this way is that you still use all the beauty of typescript classes  it gives you nice intellisense  the singleton logic keeps someway separated and it s easy to remove if needed

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I am surprised not to see the following pattern here  which actually looks very simple      shout ts class ShoutSingleton     helloWorld     return  hi        export let Shout   new ShoutSingleton      Usage  import   Shout   from    shout   Shout helloWorld

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i think maybe use generics be batter  class Singleton lt T gt       public static Instance lt T gt  c   new    T       T          if  this  instance    null               this  instance   new c                      return this  instance             private static  instance   null      how to use  step1  class MapManager extends Singleton lt MapManager gt          do something      public init   void    do       step2      MapManager Instance MapManager  init

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In Typescript  one doesn t necessarily have to follow the new instance   Singleton methodology  An imported  constructor-less static class can work equally as well   Consider   export class YourSingleton       public static foo bar      public static initialise  initVars any  void        YourSingleton foo    initvars foo           public static doThing   bar        return YourSingleton foo          You can import the class and refer to YourSingleton doThing   in any other class  But remember  because this is a static class  it has no constructor so I usually use an intialise   method that is called from a class that imports the Singleton   import  YourSingleton  from  singleton ts    YourSingleton initialise params   let  result bar   YourSingleton doThing      Don t forget that in a static class  every method and variable needs to also be static so instead of this you would use the full class name YourSingleton

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Not a pure singleton  initialization may be not lazy   but similar pattern with help of namespaces   namespace MyClass       class  MyClass                         export const instance   MyClass   new  MyClass        Access to object of Singleton   MyClass instance

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I have found a new version of this that the Typescript compiler is totally okay with  and I think is better because it doesn t require calling a getInstance   method constantly   import express    Application   from  express    export class Singleton        Define your props here   private  express  Application   express      private static  instance  Singleton     constructor         if  Singleton  instance          return Singleton  instance                You don t have an instance  so continue         Remember  to set the  instance property     Singleton  instance   this          This does come with a different drawback  If your Singleton does have any properties  then the Typescript compiler will throw a fit unless you initialize them with a value  That s why I included an  express property in my example class because unless you initialize it with a value  even if you assign it later in the constructor  Typescript will think it hasn t been defined  This could be fixed by disabling strict mode  but I prefer not to if possible  There is also another downside to this method I should point out  because the constructor is actually getting called  each time it does another instance is technically created  but not accessible  This could  in theory  cause memory leaks

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Here is yet another way to do it with a more conventional javascript approach using an IFFE    module App Counter       export var Instance         gt            var i   0          return               increment      void   gt                    i                               getCount      number   gt                    return i                                       module App       export function countStuff             App Counter Instance increment            App Counter Instance increment            alert App Counter Instance getCount              App countStuff      View a demo

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After scouring this thread and playing around with all the options above - I settled with a Singleton that can be created with proper constructors   export default class Singleton     private static  instance  Singleton    public static get instance    Singleton       return Singleton  instance        constructor    args  string             Initial setup      Singleton  instance   this        work        example           It would require an initial setup  in main ts  or index ts   which can easily be implemented by new Singleton    PARAMS      Then  anywhere in your code  just call Singleton instnace  in this case  to get work done  I would call Singleton instance work

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Since TS 2 0  we have the ability to define visibility modifiers on constructors  so now we can do singletons in TypeScript just like we are used to from other languages   Example given   class MyClass       private static  instance  MyClass       private constructor                                  public static get Instance                    Do you need arguments  Make it a regular static method instead          return this  instance     this  instance   new this              const myClassInstance   MyClass Instance      Thank you  Drenai for pointing out that if you write code using the raw compiled javascript you will not have protection against multiple instantiation  as the constraints of TS disappears and the constructor won t be hidden

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Add the following 6 lines to any class to make it  Singleton    class MySingleton       private constructor                   private static  instance  MySingleton      public static getInstance    MySingleton               return this  instance     this  instance   new this                Test example   var test   MySingleton getInstance       will create the first instance var test2   MySingleton getInstance       will return the first instance alert test     test2      true    Edit   Use Alex answer if you prefer to get the instance through a property rather a method

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Another option is to use Symbols in your module  This way you can protect your class  also if the final user of your API is using normal Javascript   let  instance   Symbol    export default class Singleton        constructor singletonToken            if  singletonToken      instance                throw new Error  Cannot instantiate directly                         Init your class            static get instance             return this  instance      this  instance    new Singleton  singleton              public myMethod   string           return  foo             Usage   var str string   Singleton instance myFoo      If the user is using your compiled API js file  also will get an error if he try to instantiate manually your class      PLAIN JAVASCRIPT   var instance   new Singleton      Error the argument singletonToken      instance symbol

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The best way I have found is   class SingletonClass        private static  instance SingletonClass   new SingletonClass         private  score number   0       constructor             if SingletonClass  instance               throw new Error  Error  Instantiation failed  Use SingletonClass getInstance   instead of new                       SingletonClass  instance   this             public static getInstance   SingletonClass               return SingletonClass  instance             public setScore value number  void               this  score   value             public getScore   number               return this  score             public addPoints value number  void               this  score    value             public removePoints value number  void               this  score -  value             Here is how you use it   var scoreManager   SingletonClass getInstance    scoreManager setScore 10   scoreManager addPoints 1   scoreManager removePoints 2   console log  scoreManager getScore        https   codebelt github io blog typescript typescript-singleton-pattern

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You can also make use of the function Object Freeze    Its simple and easy    class Singleton      instance  any   null    data  any         store data in here    constructor         if   this instance          this instance   this            return this instance        const singleton  Singleton   new Singleton    Object freeze singleton    export default singleton

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The following approach creates a Singleton class that can be used exacly like a conventional class   class Singleton       private static instance  Singleton        Assign  new Singleton    here to avoid lazy initialisation      constructor             if  Singleton instance                return Singleton instance                     this  member   0          Singleton instance   this             member  number      Each new Singleton   operation will return the same instance  This can however be unexpected by the user   The following example is more transparent to the user but requires a different usage   class Singleton       private static instance  Singleton        Assign  new Singleton    here to avoid lazy initialisation      constructor             if  Singleton instance                throw new Error  Error - use Singleton getInstance                        this member   0             static getInstance    Singleton           Singleton instance   Singleton instance    new Singleton            return Singleton instance             member  number      Usage  var obj   Singleton getInstance

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After implementing a classic pattern like class Singleton     private instance  Singleton       private constructor         public getInstance         if   this instance           this instance   new Singleton              return this instance         I realized it s pretty useless in case you want some other class to be a singleton too  It s not extendable  You have to write that singleton stuff for every class you want to be a singleton  Decorators for the rescue   singleton class MyClassThatIsSingletonToo     You can write decorator by yourself or take some from npm  I found this proxy-based implementation from  keenondrums singleton package neat enough

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You can use class expressions for this  as of 1 6 I believe    var x   new  class              lots of singleton logic            public someMethod                   or with the name if your class needs to access its type internally  var x   new  class Singleton              lots of singleton logic            public someMethod    Singleton                 Another option is to use a local class inside of your singleton using some static members  class Singleton        private static  instance      public static get instance              class InternalSingleton               someMethod                      more singleton logic                    if  Singleton  instance                Singleton  instance   new InternalSingleton                       return  lt InternalSingleton gt Singleton  instance           var x   Singleton instance  x someMethod

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This is the simplest way  class YourSingletoneClass     private static instance  YourSingletoneClass     private constructor public ifYouHaveAnyParams  string           static getInstance         if  YourSingletoneClass instance          YourSingletoneClass instance   new YourSingletoneClass  If you have any params              return YourSingletoneClass instance

[singleton] How to define Singleton in TypeScript

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