Create a new object from type parameter in generic class

Question

I m trying to create a new object of a type parameter in my generic class  In my class View  I have 2 lists of objects of generic type passed as type parameters  but when I try to make new TGridView    TypeScript says      Could not find symbol  TGridView   This is the code   module AppFW          Represents a view     export class View lt TFormView extends FormView  TGridView extends GridView gt               The list of forms          public Forms     idForm  string   TFormView                      The list of grids         public Grids     idForm  string   TGridView                   public AddForm formElement  HTMLFormElement  dataModel  any  submitFunction    e  SubmitFormViewEvent    gt  boolean   FormView               var newForm  TFormView   new TFormView formElement  dataModel  submitFunction               this Forms formElement id    newForm              return newForm                     public AddGrid element  HTMLDivElement  gridOptions  any   GridView               var newGrid  TGridView   new TGridView element  gridOptions               this Grids element id    newGrid              return newGrid                      Can I create objects from a generic type

User · Accepted Answer

Because the compiled JavaScript has all the type information erased, you can't use T to new up an object.

You can do this in a non-generic way by passing the type into the constructor.

class TestOne {
    hi() {
        alert('Hi');
    }
}

class TestTwo {
    constructor(private testType) {

    }
    getNew() {
        return new this.testType();
    }
}

var test = new TestTwo(TestOne);

var example = test.getNew();
example.hi();

You could extend this example using generics to tighten up the types:

class TestBase {
    hi() {
        alert('Hi from base');
    }
}

class TestSub extends TestBase {
    hi() {
        alert('Hi from sub');
    }
}

class TestTwo<T extends TestBase> {
    constructor(private testType: new () => T) {
    }

    getNew() : T {
        return new this.testType();
    }
}

//var test = new TestTwo<TestBase>(TestBase);
var test = new TestTwo<TestSub>(TestSub);

var example = test.getNew();
example.hi();

User · Answer

All type information is erased in JavaScript side and therefore you can t new up T just like  Sohnee states  but I would prefer having typed parameter passed in to constructor   class A      class B lt T gt        Prop  T      constructor TCreator    new     T               this Prop   new TCreator             var test   new B lt A gt  A

User · Answer

I know late but  TadasPa s answer can be adjusted a little by using  TCreator  new     gt  T   instead of  TCreator    new     T      so the result should look like this  class A      class B lt T gt        Prop  T      constructor TCreator  new     gt  T            this Prop   new TCreator             var test   new B lt A gt  A

User · Answer

I m late for the party but this is the way I got it working  For arrays we need do some tricks      public clone lt T gt  sourceObj  T   T         var cloneObj  T      as T        for  var key in sourceObj             if  sourceObj key  instanceof Array                if  sourceObj key                       create an empty value first                let str  string          key                             Object assign cloneObj  JSON parse str                     update with the real value                cloneObj key    sourceObj key                 else                  Object assign cloneObj                               else if  typeof sourceObj key       object                 cloneObj key    this clone sourceObj key               else               if  cloneObj hasOwnProperty key                    cloneObj key    sourceObj key                 else      insert the property                   need create a JSON to use the  key  as its value                let str  string          key             sourceObj key                            insert the new field                Object assign cloneObj  JSON parse str                                          return cloneObj         Use it like this     let newObj  SomeClass   clone lt SomeClass gt  someClassObj     It can be improved but worked for my needs

User · Answer

This is what I do to retain type info   class Helper      public static createRaw lt T gt  TCreator    new     T     data  any   T           return Object assign new TCreator    data           public static create lt T gt  TCreator    new     T     data  T   T            return this createRaw TCreator  data                it  create helper        gt        class A           public data  string            class B           public data  string          public getData    string               return this data                      var str    foobar        var a1   Helper create lt A gt  A   data  str        expect a1 instanceof A  toBeTruthy        expect a1 data  toBe str        var a2   Helper create A   data  str        expect a2 instanceof A  toBeTruthy        expect a2 data  toBe str        var b1   Helper createRaw B   data  str        expect b1 instanceof B  toBeTruthy        expect b1 data  toBe str       expect b1 getData    toBe str

User · Answer

Here is example if you need parameters in constructor  class Sample       public innerField  string       constructor data  Partial lt Sample gt             this innerField   data innerField           export class GenericWithParams lt TType gt        public innerItem  TType       constructor data  Partial lt GenericWithParams lt TType gt  gt   private typePrototype  new  i  Partial lt TType gt     gt  TType            this innerItem   this factoryMethodOnModel data innerItem              private factoryMethodOnModel    item  Partial lt TType gt    TType   gt            return new this typePrototype item             const instance   new GenericWithParams lt Sample gt    innerItem     innerField   test      Sample

User · Answer

Not quite answering the question  but  there is a nice library for those kind of problems  https   github com typestack class-transformer  although it won t work for generic types  as they don t really exists at run-time  here all work is done with class names  which are classes constructors     For instance   import  Type  plainToClass  deserialize  from  class-transformer    export class Foo        Type Bar      public nestedClass  Bar       public someVar  string       public someMethod    string               return this nestedClass someVar   this someVar           export class Bar       public someVar  string     const json      someVar    a    nestedClass     someVar    B      const optionA   plainToClass Foo  JSON parse json    const optionB   deserialize Foo  json    optionA someMethod       works optionB someMethod       works

User · Answer

To create a new object within generic code  you need to refer to the type by its constructor function   So instead of writing this   function activatorNotWorking lt T extends IActivatable gt  type  T   T       return new T       compile error could not find symbol T     You need to write this   function activator lt T extends IActivatable gt  type    new    T       T       return new type       var classA  ClassA   activator ClassA     See this question  Generic Type Inference with Class Argument

User · Answer

I m adding this by request  not because I think it directly solves the question  My solution involves a table component for displaying tables from my SQL database  export class TableComponent lt T gt         public Data  T              public constructor          protected type  new  value  Partial lt T gt     gt  T                protected insertRow value  Partial lt T gt    void           let row  T   new this type value           this Data push row            To put this to use  assume I have a view  or table  in my database VW MyData and I want to hit the constructor of my VW MyData class for every entry returned from a query  export class MyDataComponent extends TableComponent lt VW MyData gt         public constructor protected service  DataService            super VW MyData           this query               protected query    void           this service post      subscribe  json  VW MyData      gt                for  let item of json                    this insertRow item                                    The reason this is desirable over simply assigning the returned value to Data  is say I have some code that applies a transformation to some column of VW MyData in its constructor  export class VW MyData            public RawColumn  string      public TransformedColumn  string        public constructor init   Partial lt VW MyData gt             Object assign this  init           this TransformedColumn   this transform this RawColumn              protected transform input  string   string           return  Transformation of   input              This allows me to perform transformations  validations  and whatever else on all my data coming in to TypeScript  Hopefully it provides some insight for someone

User · Answer

I was trying to instantiate the generic from within a base class  None of the above examples worked for me as they required a concrete type in order to call the factory method   After researching for awhile on this and unable to find a solution online  I discovered that this appears to work     protected activeRow  T      as T    The pieces    activeRow  T       lt -- activeRow now equals a new object            as T   lt -- As the type I specified     All together   export abstract class GridRowEditDialogBase lt T extends DataRow gt  extends DialogBase         protected activeRow  T      as T       That said  if you need an actual instance you should use   export function getInstance lt T extends Object gt  type   new     args  any      gt  T      args  any     T         return new type    args       export class Foo     bar         console log  Hello World         getInstance Foo  bar      If you have arguments  you can use   export class Foo2     constructor public arg1  string  public arg2  number            bar         console log this arg1       console log this arg2         getInstance Foo   Hello World   2  bar

User · Answer

i use this  let instance    lt T gt     it generally works EDIT 1   export class EntityCollection lt T extends   id  number   gt     mutable  EditableEntity lt T gt            immutable  T           edit index  number        this mutable index  entity   Object assign  lt T gt     this immutable index

User · Answer

export abstract class formBase lt T gt  extends baseClass      protected item      as T      Its object will be able to receive any parameter  however  type T is only a typescript reference and can not be created through a constructor  That is  it will not create any class objects

[typescript] Create a new object from type parameter in generic class

Examples related to typescript

Examples related to generics