Get properties of a class

Question

Is there a way to get properties names of class in TypeScript   In the example  I would like to  describe  the class A or any class and get an array of its properties  maybe only public ones    is it possible  Or should I instantiate the object first   class A       private a1      private a2          Getters and Setters        class Describer lt E gt        toBeDescribed E       describe    Array lt string gt                           Do something with  toBeDescribed                                                return   a1    a2       lt - Example          let describer   new Describer lt A gt     let x  describer describe        x should be   a1    a2

User · Answer

Use these  export class TableColumns lt T gt       constructor private t  new      gt  T            var fields  string     Object keys new t             console log  fields   fields          console log  t   t             Usage   columns logs   new TableColumns lt LogItem gt  LogItem     Output   fields  12     id    code    source    title    deleted    checked    body    json    dt insert    dt checked    screenshot    uid     js class  t class LogItem   constructor         this id   0      this code   0      this source           this title           this deleted   false      this checked   false

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I am currently working on a Linq-like library for Typescript and wanted to implement something like GetProperties of C  in Typescript   Javascript  The more I work with Typescript and generics  the clearer picture I get of that you usually have to have an instantiated object with intialized properties to get any useful information out at runtime about properties of a class  But it would be nice to retrieve information anyways just from the constructor function object  or an array of objects and be flexible about this    Here is what I ended up with for now    First off  I define Array prototype method   extension method  for you C  developers     export       creating a module of below code declare global     interface Array lt T gt        GetProperties lt T gt  TClass  Function  sortProps  boolean   string          The GetProperties method then looks like this  inspired by madreason s answer   if   Array prototype GetProperties      Array prototype GetProperties   function  lt T gt  TClass  any   null  sortProps  boolean   false   string         if  TClass     null    TClass     undefined          if  this     null    this     undefined    this length     0            return       not possible to find out more information - return empty array                      debugger     if  TClass     null  amp  amp  TClass     undefined          if  this     null  amp  amp  this     undefined            if  this length  gt  0              let knownProps  string     Describer describe this 0   Where x   gt  x     null  amp  amp  x     undefined             if  sortProps  amp  amp  knownProps     null  amp  amp  knownProps     undefined                knownProps   knownProps OrderBy p   gt  p                         return knownProps                    if  TClass     null  amp  amp  TClass     undefined              let knownProps  string     Describer describe TClass  Where x   gt  x     null  amp  amp  x     undefined             if  sortProps  amp  amp  knownProps     null  amp  amp  knownProps     undefined                knownProps   knownProps OrderBy p   gt  p                         return knownProps                              return       give up           The describer method is about the same as madreason s answer  It can handle both class Function and if you get an object instead  It will then use Object getOwnPropertyNames if no class Function is given  i e  the class  type  for C  developers    class Describer     private static FRegEx   new RegExp     this              g     static describe val  any  parent   false   string         let isFunction   Object prototype toString call val       object Function        if  isFunction          let result             if  parent            var proto   Object getPrototypeOf val prototype           if  proto              result   result concat this describe proto constructor  parent                            result   result concat val toString   match this FRegEx          result   result Where r   gt  r     null  amp  amp  r     undefined         return result            else         if  typeof val     object             let knownProps  string     Object getOwnPropertyNames val           return knownProps                    return val     null    val tostring                  Here you see two specs for testing this out with Jasmine   class Hero     name  string    gender  string    age  number    constructor name  string       gender  string       age  number   0        this name   name      this gender   gender      this age   age         class HeroWithAbility extends Hero     ability  string    constructor ability  string             super        this ability   ability         describe  Array Extensions tests for TsExtensions Linq esque library        gt       it  can retrieve props for a class items of an array        gt        let heroes  Hero       lt Hero gt   name   Han Solo   age  44  gender   M      lt Hero gt   name   Leia   age  29  gender   F      lt Hero gt   name   Luke   age  24  gender   M      lt Hero gt   name   Lando   age  47  gender   M          let foundProps   heroes GetProperties Hero  false         debugger     let expectedArrayOfProps     name    age    gender        expect foundProps  toEqual expectedArrayOfProps       expect heroes GetProperties Hero  true   toEqual   age    gender    name              it  can retrieve props for a class only knowing its function        gt        let heroes  Hero             let foundProps   heroes GetProperties Hero  false       let expectedArrayOfProps     this name    this gender    this age        expect foundProps  toEqual expectedArrayOfProps       let foundPropsThroughClassFunction   heroes GetProperties Hero  true         debugger     expect foundPropsThroughClassFunction SequenceEqual   this age    this gender    this name     toBe true           And as madreason mentioned  you have to initialize the props to get any information out from just the class Function itself  or else it is stripped away when Typescript code is turned into Javascript code   Typescript 3 7 is very good with Generics  but coming from a C  and Reflection background  some fundamental parts of Typescript and generics still feels somewhat loose and unfinished business  Like my code here  but at least I got out the information I wanted - a list of property names for a given class or instance of objects    SequenceEqual is this method btw       if   Array prototype SequenceEqual      Array prototype SequenceEqual   function  lt T gt  compareArray  T   boolean       if   Array isArray this      Array isArray compareArray     this length     compareArray length        return false      var arr1   this concat   sort        var arr2   compareArray concat   sort        for  var i   0  i  lt  arr1 length  i            if  arr1 i      arr2 i           return false            return true

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Just for fun  class A       private a1   void 0      private a2   void 0     class B extends A       private a3   void 0      private a4   void 0     class C extends B       private a5   void 0      private a6   void 0     class Describer       private static FRegEx   new RegExp     this              g        static describe val  Function  parent   false   string             var result               if  parent                var proto   Object getPrototypeOf val prototype               if  proto                    result   result concat this describe proto constructor  parent                                     result   result concat val toString   match this FRegEx                  return result           console log Describer describe A         this a1    this a2   console log Describer describe B         this a3    this a4   console log Describer describe C  true         this a1         this a6     Update  If you are using custom constructors  this functionality will break

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There is another answer here that also fits the authors request    39 compile-time  39  way to get all property names defined interface  If you use the plugin ts-transformer-keys and an Interface to your class you can get all the keys for the class   But if you re using Angular or React then in some scenarios there is additional configuration necessary  webpack and typescript  to get it working  https   github com kimamula ts-transformer-keys issues 4

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Other answers mainly get all name of object  to get value of property  you can use yourObj name   for example   var propNames   Object getOwnPropertyNames yourObj   propNames forEach      function propName            console log              name      propName              value      yourObj propName

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This TypeScript code  class A       private a1      public a2      compiles to this JavaScript code  class A       That s because properties in JavaScript start extisting only after they have some value  You have to assign the properties some value   class A       private a1           public a2           it compiles to  class A       constructor             this a1               this a2                 Still  you cannot get the properties from mere class  you can get only methods from prototype   You must create an instance  Then you get the properties by calling Object getOwnPropertyNames     let a   new A    let array   return Object getOwnPropertyNames a    array 0       a1   array 1       a2     Applied to your example  class Describer       static describe instance   Array lt string gt            return Object getOwnPropertyNames instance            let a   new A    let x   Describer describe a

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Another solution  You can just iterate over the object keys like so   Note  you must use an instantiated object with existing properties   printTypeNames lt T gt  obj  T        const objectKeys   Object keys obj  as Array lt keyof T gt       for  let key of objectKeys               console log  key     key

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Some answers are partially wrong  and some facts in them are partially wrong as well   Answer your question  Yes  You can   In Typescript  class A       private a1      private a2        Generates the following code in Javascript   var A        class     function          function A               return A          as  Erik Cupal said  you could just do   let a   new A    let array   return Object getOwnPropertyNames a     But this is incomplete  What happens if your class has a custom constructor  You need to do a trick with Typescript because it will not compile  You need to assign as any   let className any   A  let a   new className      the members will have value undefined   A general solution will be   class A       private a1      private a2      constructor a1 number  a2 string           this a1   a1          this a2   a2           class Describer      describeClass  typeOfClass any          let a   new typeOfClass           let array   Object getOwnPropertyNames a          return array   you can apply any filter here          For better understanding this will reference depending on the context

[typescript] Get properties of a class

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