TypeScript and field initializers

Question

How to init a new class in TS in such a way  example in C  to show what I want           some code before return new MyClass   Field1    ASD   Field2    QWE             some code after    edit  When I was writing this question I was pure  NET developer without much of JS knowledge  Also TypeScript was something completely new  announced as new C -based superset of JavaScript  Today I see how stupid this question was     Anyway if anyone still is looking for an answer  please  look at the possible solutions below   First thing to note is in TS we shouldn t create empty classes for models  Better way is to create interface or type  depending on needs   Good article from Todd Motto  https   ultimatecourses com blog classes-vs-interfaces-in-typescript  SOLUTION 1   type MyType     prop1  string  prop2  string     return  lt MyType gt    prop1      prop2          SOLUTION 2   type MyType     prop1  string  prop2  string     return   prop1      prop2       as MyType    SOLUTION 3  when you really need a class    class MyClass      constructor public data    prop1  string  prop2  string                return new MyClass   prop1      prop2           or   class MyClass      constructor public prop1  string  public prop2  string              return new MyClass            Of course in both cases you may not need casting types manually because they will be resolved from function method return type

User · Answer

I wanted a solution that would have the following:

All the data objects are required and must be filled by the constructor.
No need to provide defaults.
Can use functions inside the class.

Here is the way that I do it:

export class Person {
  id!: number;
  firstName!: string;
  lastName!: string;

  getFullName() {
    return `${this.firstName} ${this.lastName}`;
  }

  constructor(data: OnlyData<Person>) {
    Object.assign(this, data);
  }
}

const person = new Person({ id: 5, firstName: "John", lastName: "Doe" });
person.getFullName();

All the properties in the constructor are mandatory and may not be omitted without a compiler error.

It is dependant on the OnlyData that filters out getFullName() out of the required properties and it is defined like so:

// based on : https://medium.com/dailyjs/typescript-create-a-condition-based-subset-types-9d902cea5b8c
type FilterFlags<Base, Condition> = { [Key in keyof Base]: Base[Key] extends Condition ? never : Key };
type AllowedNames<Base, Condition> = FilterFlags<Base, Condition>[keyof Base];
type SubType<Base, Condition> = Pick<Base, AllowedNames<Base, Condition>>;
type OnlyData<T> = SubType<T, (_: any) => any>;

Current limitations of this way:

Requires TypeScript 2.8
Classes with getters/setters

User · Answer

Below is a solution that combines a shorter application of Object assign to more closely model the original C  pattern   But first  lets review the techniques offered so far  which include    Copy constructors that accept an object and apply that to Object assign A clever Partial lt T gt  trick within the copy constructor Use of  casting  against a POJO Leveraging Object create instead of Object assign   Of course  each have their pros cons   Modifying a target class to create a copy constructor may not always be an option   And  casting  loses any functions associated with the target type   Object create seems less appealing since it requires a rather verbose property descriptor map   Shortest  General-Purpose Answer  So  here s yet another approach that is somewhat simpler  maintains the type definition and associated function prototypes  and more closely models the intended C  pattern   const john   Object assign  new Person          name   John       age  29      address   Earth        That s it   The only addition over the C  pattern is Object assign along with 2 parenthesis and a comma   Check out the working example below to confirm it maintains the type s function prototypes   No constructors required  and no clever tricks   Working Example  This example shows how to initialize an object using an approximation of a C  field initializer    x000D   x000D  class Person   x000D      name  string       x000D      address  string       x000D      age  number   0  x000D   x000D      aboutMe     x000D          return  Hi  I m   this name   aged   this age  and from   this address    x000D        x000D    x000D   x000D     typescript field initializer  maintains  type  definition  x000D  const john   Object assign  new Person      x000D      name   John   x000D      age  29  x000D      address   Earth  x000D      x000D   x000D     initialized object maintains aboutMe   function prototype x000D  console log  john aboutMe      x000D   x000D   x000D

User · Answer

To init a class without redeclaring all the properties for defaults  class MyClass     prop1   string    required to be passed in   prop2   string    required to be passed in   prop3    some default    prop4   123    constructor opts  prop1 string  prop2 string   amp  Partial lt MyClass gt        Object assign this opts         This combines some of the already excellent answers

User · Answer

Here s a solution that   doesn t force you to make all fields optional  unlike Partial lt     gt   differentiates between class methods and fields of function type  unlike the OnlyData lt     gt  solution  provides a nice structure by defining a Params interface doesn t need to repeat variable names  amp  types more than once  The only drawback is that it looks more complicated at first      Define all fields here interface PersonParams     id  string   name   string   coolCallback       gt  string       extend the params interface with an interface that has    the same class name as the target class     if you omit the Params interface  you will have to redeclare    all variables in the Person class  interface Person extends PersonParams         merge the Person interface with Person class  no need to repeat params     person will have all fields of PersonParams     yes  this is valid TS  class Person     constructor params  PersonParams           could also do Object assign this  params        this id   params id      this name   params name          intellisence will expect params        to have  coolCallback  but not  sayHello      this coolCallback   params coolCallback            compatible with functions   sayHello         console log  Hi   this name                you can only export on another line  not  export default class      export default Person

User · Answer

Updated 07 12 2016  Typescript 2 1 introduces Mapped Types and provides Partial lt T gt   which allows you to do this       class Person       public name  string    default      public address  string    default      public age  number   0       public constructor init  Partial lt Person gt             Object assign this  init            let persons         new Person        new Person          new Person  name  John         new Person  address  Earth             new Person  age 20  address  Earth   name  John          Original Answer   My approach is to define a separate fields variable that you pass to the constructor  The trick is to redefine all the class fields for this initialiser as optional  When the object is created  with its defaults  you simply assign the initialiser object onto this   export class Person       public name  string    default      public address  string    default      public age  number   0       public constructor          fields                 name   string              address   string              age   number                      if  fields  Object assign this  fields             or do it manually  bit more safe     if  fields        this name   fields name    this name             this address   fields address    this address              this age   fields age    this age              usage   let persons         new Person        new Person  name  Joe         new Person           name  Joe           address  planet Earth              new Person           age 5                         address  planet Earth           name  Joe              new Person new Person  name  Joe       shallow clone       and console output   Person   name   default   address   default   age  0   Person   name   Joe   address   default   age  0   Person   name   Joe   address   planet Earth   age  0   Person   name   Joe   address   planet Earth   age  5   Person   name   Joe   address   default   age  0        This gives you basic safety and property initialization  but its all optional and can be out-of-order  You get the class s defaults left alone if you don t pass a field    You can also mix it with required constructor parameters too -- stick fields on the end    About as close to C  style as you re going to get I think  actual field-init syntax was rejected   I d much prefer proper field initialiser  but doesn t look like it will happen yet    For comparison  If you use the casting approach  your initialiser object must have ALL the fields for the type you are casting to  plus don t get any class specific functions  or derivations  created by the class itself

User · Answer

You can affect an anonymous object casted in your class type  Bonus  In visual studio  you benefit of intellisense this way     var anInstance  AClass    lt AClass gt        Property1   Value       Property2   Value       PropertyBoolean  true      PropertyNumber  1      Edit   WARNING If the class has methods  the instance of your class will not get them   If AClass has a constructor  it will not be executed   If you use instanceof AClass  you will get false   In conclusion  you should used interface and not class  The most common use is for the domain model declared as Plain Old Objects  Indeed  for domain model you should better use interface instead of class  Interfaces are use at compilation time for type checking and unlike classes  interfaces are completely removed during compilation   interface IModel      Property1  string     Property2  string     PropertyBoolean  boolean     PropertyNumber  number     var anObject  IModel          Property1   Value        Property2   Value        PropertyBoolean  true       PropertyNumber  1

User · Answer

This is another solution   return     Field1    ASD     Field2    QWE     as myClass

User · Answer

if you want to create new instance without set initial value when instance   1- you have to use class not interface   2- you have to set initial value when create class   export class IStudentDTO    Id         number   0   Name       string         student  IStudentDTO   new IStudentDTO

User · Answer

In some scenarios it may be acceptable to use Object create  The Mozilla reference includes a polyfill if you need back-compatibility or want to roll your own initializer function   Applied to your example   Object create Person prototype         Field1     value   ASD          Field2     value   QWE          Useful Scenarios   Unit Tests Inline declaration   In my case I found this useful in unit tests for two reasons    When testing expectations I often want to create a slim object as an expectation Unit test frameworks  like Jasmine  may compare the object prototype    proto    and fail the test  For example    var actual   new MyClass    actual field1    ASD   expect   field1   ASD     toEqual actual      fails     The output of the unit test failure will not yield a clue about what is mismatched    In unit tests I can be selective about what browsers I support   Finally  the solution proposed at http   typescript codeplex com workitem 334 does not support inline json-style declaration   For example  the following does not compile    var o        m  MyClass    Field1  ASD

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If you re using an old version of typescript  lt  2 1 then you can use similar to the following which is basically casting of any to typed object   const typedProduct    lt Product gt                       code   lt string gt product sku                         NOTE  Using this method is only good for data models as it will remove   all the methods in the object  It s basically casting any object to a   typed object

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You could have a class with optional fields  marked with    and a constructor that receives an instance of the same class   class Person       name  string         required     address   string     optional     age   number         optional      constructor person  Person            Object assign this  person            let persons         new Person   name   John          new Person   address   Earth              new Person   age  20  address   Earth   name   John           In this case  you will not be able to omit the required fields  This gives you fine-grained control over the object construction   You could use the constructor with the Partial type as noted in other answers   public constructor init  Partial lt Person gt         Object assign this  init       The problem is that all fields become optional and it is not desirable in most cases

User · Answer

I suggest an approach that does not require Typescript 2 1  class Person       public name  string      public address   string      public age  number       public constructor init Person            Object assign this  init              public someFunc                todo          let person   new Person  lt Person gt   age 20  name  quot John quot      person someFunc     key points   Typescript 2 1 not required  Partial lt T gt  not required It supports functions  in comparison with simple type assertion which does not support functions

User · Answer

The easiest way to do this is with type casting   return  lt MyClass gt   Field1   ASD   Field2   QWE

User · Answer

I d be more inclined to do it this way  using  optionally  automatic properties and defaults  You haven t suggested that the two fields are part of a data structure  so that s why I chose this way    You could have the properties on the class and then assign them the usual way  And obviously they may or may not be required  so that s something else too  It s just that this is such nice syntactic sugar    class MyClass      constructor public Field1 string       public Field2 string                        other constructor stuff          var myClass   new MyClass  ASD    QWE    alert myClass Field1      voila  statement completion on these properties

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Update  Since writing this answer  better ways have come up  Please see the other answers below that have more votes and a better answer  I cannot remove this answer since it s marked as accepted     Old answer  There is an issue on the TypeScript codeplex that describes this  Support for object initializers   As stated  you can already do this by using interfaces in TypeScript instead of classes   interface Name       first  string      last  string    class Person       name  Name      age  number     var bob  Person         name            first   Bob           last   Smith              age  35

[typescript] TypeScript and field initializers

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