How to define static property in TypeScript interface

Question

I just want to declare a static property in typescript interface  I have not found anywhere regarding this   interface myInterface     static Name string      Is it possible

User · Answer

Another option not mentioned here is defining variable with a type representing static interface and assigning to it class expression:

interface MyType {
    instanceMethod(): void;
}

interface MyTypeStatic {
    new(): MyType;
    staticMethod(): void;
}

// ok
const MyTypeClass: MyTypeStatic = class MyTypeClass {
    public static staticMethod() { }
    instanceMethod() { }
}

// error: 'instanceMethod' is missing
const MyTypeClass1: MyTypeStatic = class MyTypeClass {
    public static staticMethod() { }
}

// error: 'staticMethod' is missing
const MyTypeClass2: MyTypeStatic = class MyTypeClass {
    instanceMethod() { }
}

The effect is same as in answer with decorators, but without overhead of decorators

Playground

Relevant suggestion/discussion on GitHub

User · Answer

I found a way to do this  without decorators  for my specific use case   The important part that checks for static members is IObjectClass and using cls  IObjectClass lt T gt  in the createObject method     ------------------------    Library   ------------------------ interface IObject     id  number    interface IObjectClass lt T gt      new    T    table name  string    function createObject lt T extends IObject gt  cls  IObjectClass lt T gt   data Partial lt T gt   T     let obj T     lt any gt Object  assign         data              id  1        table name  cls table name              return obj       ------------------------    Implementation   ------------------------ export class User implements IObject     static table name  string    user     id  number    name  string       ------------------------    Application   ------------------------ let user   createObject User   name   Jimmy     console log user name

User · Answer

You can merge interface with namespace using the same name   interface myInterface      namespace myInterface     Name string      But this interface is only useful to know that its have property Name  You can not implement it

User · Answer

duncan s solution above specifying new   for the static type works also with interfaces   interface MyType       instanceMethod       interface MyTypeStatic       new   MyType      staticMethod

User · Answer

You can define interface normally  interface MyInterface       Name string     but you can t just do class MyClass implements MyInterface       static Name string     typescript won t care about this field     Name string             and demand this one instead    To express that a class should follow this interface for its static properties you need a bit of trickery  var MyClass  MyInterface  MyClass   class       static Name string     if the class doesn t have that field it won t compile    You can even keep the name of the class  TypeScript  2 0  won t mind  var MyClass  MyInterface  MyClass   class MyClass       static Name string     if the class doesn t have that field it won t compile    If you want to inherit from many interfaces statically you ll have to merge them first into a new one  interface NameInterface       Name string    interface AddressInterface       Address string    interface NameAndAddressInterface extends NameInterface  AddressInterface     var MyClass  NameAndAddressInterface  MyClass   class MyClass       static Name string     if the class doesn t have that static field code won t compile     static Address string     if the class doesn t have that static field code won t compile    Or if you don t want to name merged interface you can do  interface NameInterface       Name string    interface AddressInterface       Address string    var MyClass  NameInterface  amp  AddressInterface  MyClass   class MyClass       static Name string     if the class doesn t have that static field code won t compile     static Address string     if the class doesn t have that static field code won t compile    Working example

User · Answer

If you re looking to define a static class  ie  all methods properties are static   you can do something like this   interface MyStaticClassInterface     foo   string     var myStaticClass MyStaticClassInterface       foo         return  bar            In this case  the static  class  is really just a plain-ol -js-object  which implements all the methods of MyStaticClassInterface

User · Answer

Static modifiers cannot appear on a type member  TypeScript error TS1070   That s why I recommend to use an abstract class to solve the mission   Example     Interface definition abstract class MyInterface     static MyName  string    abstract getText    string        Interface implementation class MyClass extends MyInterface     static MyName    TestName     getText    string       return  This is my name static name    MyClass MyName                Test run const test  MyInterface   new MyClass    console log test getText

User · Answer

Static properties are usually placed on the  global  constructor for the object  whereas the  interface  keyword applies to instances of the object   The previous answer given is of course correct if you are writing the class in TypeScript   It may help others to know that if you are describing an object that is already implemented elsewhere  then the global constructor including static properties can be declared like this   declare var myInterface       new    Interface    Name string

User · Answer

Though static keyword not supported in interface in Typescript  but we can achieve it by creating a function interface that has static member   In my following code I have created a function interface Factory that has two static members serialNumber and printSerial      factory is a function interface interface Factory lt T gt         name  string  age  number   T         staic property     serialNumber  number         static method     printSrial       gt  void     class Dog       constructor public name  string  public age  number         const dogFactory  Factory lt Dog gt     name  age    gt        return new Dog name  age         initialising static members  dogFactory serialNumber   1234  dogFactory printSrial        gt  console log dogFactory serialNumber       instance of Dog that DogFactory creates const myDog   dogFactory  spike   3      static property that returns 1234 console log dogFactory serialNumber     static method that prints the serial 1234 dogFactory printSrial

User · Answer

I implemented a solution like Kamil Szot s  and it has an undesired effect  I have not enough reputation to post this as a comment  so I post it here in case someone is trying that solution and reads this  The solution is  interface MyInterface       Name  string     const MyClass   class       static Name  string      However  using a class expression doesn t allow me to use MyClass as a type  If I write something like  const myInstance  MyClass   myInstance turns out to be of type any  and my editor shows the following error   MyClass  refers to a value  but is being used as a type here  Did you mean  typeof MyClass  ts 2749   I end up losing a more important typing than the one I wanted to achieve with the interface for the static part of the class  Val s solution using a decorator avoids this pitfall

User · Answer

The other solutions seem to stray from the blessed path and I found that my scenario was covered in the Typescript documentation which I ve paraphrased below  interface AppPackageCheck lt T gt      new  packageExists  boolean   T   checkIfPackageExists    boolean     class WebApp       public static checkIfPackageExists    boolean           return false             constructor public packageExists  boolean        class BackendApp       constructor public packageExists  boolean        function createApp lt T gt  type  AppPackageCheck lt T gt    T       const packageExists   type checkIfPackageExists        return new type packageExists     let web   createApp WebApp       compiler failure here  missing checkIfPackageExists let backend   createApp BackendApp

User · Answer

Yes  it is possible  Here is the solution  export interface Foo        test    void     export namespace Foo        export function statMethod    void           console log 2

User · Answer

You can t define a static property on an interface in TypeScript   Say you wanted to change the Date object  rather than trying to add to the definitions of Date  you could wrap it  or simply create your rich date class to do the stuff that Date doesn t do   class RichDate       public static MinValue   new Date        Because Date is an interface in TypeScript  you can t extend it with a class using the extends keyword  which is a bit of a shame as this would be a good solution if date was a class   If you want to extend the Date object to provide a MinValue property on the prototype  you can   interface Date       MinValue  Date     Date prototype MinValue   new Date 0     Called using   var x   new Date    console log x MinValue     And if you want to make it available without an instance  you also can    but it is a bit fussy   interface DateStatic extends Date       MinValue  Date     Date  MinValue     new Date 0     Called using   var x  DateStatic    lt any gt Date     We aren t using an instance console log x MinValue

User · Answer

Follow  Duncan s  Bartvds s answer  here to provide a workable way after years passed  At this point after Typescript 1 5 released   Jun 15  15   your helpful interface interface MyType       instanceMethod       interface MyTypeStatic       new   MyType      staticMethod       can be implemented this way with the help of decorator     class decorator    function staticImplements lt T gt          return  lt U extends T gt  constructor  U    gt   constructor       staticImplements lt MyTypeStatic gt         this statement implements both normal interface  amp  static interface    class MyTypeClass      implements MyType         so this become optional not required        public static staticMethod          instanceMethod         Refer to my comment at github issue 13462  visual result  Compile error with a hint of static method missing   After static method implemented  hint for method missing   Compilation passed after both static interface and normal interface fulfilled

[typescript] How to define static property in TypeScript interface

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