Get keys of a Typescript interface as array of strings

Question

I ve a lot of tables in Lovefield and their respective Interfaces for what columns they have  Example  export interface IMyTable     id  number    title  string    createdAt  Date    isDeleted  boolean     I d like to have the property names of this interface in an array like this  const IMyTable     quot id quot    quot title quot    quot createdAt quot    quot isDeleted quot     I cannot make an object array based on the interface IMyTable directly which should do the trick because I d be getting the interface names of the tables dynamically  Hence I need to iterate over these properties in the interface and get an array out of it  How do I achieve this result

User · Answer

Instead of defining IMyTable as in interface  try defining it as a class   In typescript you can use a class like an interface   So for your example  define generate your class like this   export class IMyTable       constructor          public id               public title               public createdAt  Date   null          public isDeleted   false           Use it as an interface   export class SomeTable implements IMyTable               Get keys   const keys   Object keys new IMyTable

User · Answer

As of TypeScript 2 3  or should I say 2 4  as in 2 3 this feature contains a bug which has been fixed in typescript 2 4-dev   you can create a custom transformer to achieve what you want to do   Actually  I have already created such a custom transformer  which enables the following   https   github com kimamula ts-transformer-keys  import   keys   from  ts-transformer-keys    interface Props     id  string    name  string    age  number    const keysOfProps   keys lt Props gt      console log keysOfProps        id    name    age     Unfortunately  custom transformers are currently not so easy to use  You have to use them with the TypeScript transformation API instead of executing tsc command  There is an issue requesting a plugin support for custom transformers

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This was a tough one   Thank you  everyone  for your assistance  My need was to get keys of an interface as an array of strings to simplify mocha chai scripting   Not concerned about using in the app  yet   so didn t need the ts files to be created   Thanks to ford04 for the assistance  his solution above was a huge help and it works perfectly  NO compiler hacks   Here s the modified code  Option 2  Code generator based on TS compiler API  ts-morph  Node Module npm install --save-dev ts-morph  keys ts NOTE   this assumes all ts files are located in the root of   src and there are no subfolders  adjust accordingly import     Project    VariableDeclarationKind    InterfaceDeclaration    from  quot ts-morph quot       initName is name of the interface file below the root    src is considered the root const Keys    intName  string   string     gt      const project   new Project      const sourceFile   project addSourceFileAtPath    src   intName  ts      const node   sourceFile getInterface intName      const allKeys   node getProperties   map  p    gt  p getName        return allKeys      export default Keys    usage import keys from  quot   keys quot    const myKeys   keys  quot MyInterface quot     ts file name without extension  console log myKeys

User · Answer

The following requires you to list the keys on your own  but at least TypeScript will enforce IUserProfile and IUserProfileKeys have the exact same keys  Required lt T gt  was added in TypeScript 2 8    export interface IUserProfile      id  string    name  string     type KeysEnum lt T gt       P in keyof Required lt T gt    true    const IUserProfileKeys  KeysEnum lt IUserProfile gt        id  true    name  true

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Maybe it s too late  but in version 2 1 of TypeScript you can use key of like this  interface Person       name  string      age  number      location  string     type K1   keyof Person      quot name quot     quot age quot     quot location quot  type K2   keyof Person         quot length quot     quot push quot     quot pop quot     quot concat quot        type K3   keyof    x  string   Person        string  Source  https   www typescriptlang org docs handbook release-notes typescript-2-1 html keyof-and-lookup-types

User · Answer

Can t  Interfaces don t exist at runtime    workaround  Create a variable of the type and use Object keys on it

User · Answer

You can t do it  Interfaces don t exist at runtime  like  basarat said    Now  I am working with following   const IMyTable id    id   const IMyTable title    title   const IMyTable createdAt    createdAt   const IMyTable isDeleted    isDeleted    export const IMyTable keys       IMyTable id    IMyTable title    IMyTable createdAt    IMyTable isDeleted      export interface IMyTable      IMyTable id   number     IMyTable title   string     IMyTable createdAt   Date     IMyTable isDeleted   boolean

User · Answer

This should work  var IMyTable  Array lt keyof IMyTable gt      id    title    createdAt    isDeleted      or  var IMyTable   keyof IMyTable        id    title    createdAt    isDeleted

User · Answer

declarations d ts export interface IMyTable         id  number        title  string        createdAt  Date        isDeleted  boolean   declare var Tes  IMyTable     call in annother page console log Tes id

User · Answer

I had a similar problem that I had a giant list of properties that I wanted to have both an interface  and an object out of it  NOTE  I didn t want to write  type with keyboard  the properties twice   Just DRY   One thing to note here is  interfaces are enforced types at compile-time  while objects are mostly run-time   Source  As  derek mentioned in another answer  the common denominator of interface and object can be a class that serves both a type and a value  So  TL DR  the following piece of code should satisfy the needs  class MyTableClass          list the propeties here  ONLY WRITTEN ONCE     id    quot  quot       title    quot  quot       isDeleted   false                                                         This is the pure interface version  to be used exported interface IMyTable extends MyTableClass                                                           Props type as an array  to be exported type MyTablePropsArray   Array lt keyof IMyTable gt       Props array itself  const propsArray  MyTablePropsArray       Object keys new MyTableClass    as MyTablePropsArray   console log propsArray      prints out     quot id quot    quot title quot    quot isDeleted quot                                                         Example of creating a pure instance as an object const tableInstance  MyTableClass        works properly      id   quot 3 quot       title   quot hi quot       isDeleted  false       Here is the above code in Typescript Playground to play more  PS  If you don t want to assign initial values to the properties in the class  and stay with the type  you can do the constructor trick  class MyTableClass          list the propeties here  ONLY WRITTEN ONCE     constructor          readonly id   string          readonly title   string          readonly isDeleted   boolean              console log Object keys new MyTableClass          prints out    quot id quot    quot title quot    quot isDeleted quot     Constructor Trick in TypeScript Playground

User · Answer

You will need to make a class that implements your interface  instantiate it and then use Object keys yourObject  to get the properties   export class YourClass implements IMyTable               then  let yourObject YourClass   new YourClass    Object keys yourObject  forEach         gt

User · Answer

Safe variants Creating an array or tuple of keys from an interface with safety compile-time checks requires a bit of creativity  Types are erased at run-time and object types  unordered  named  cannot be converted to tuple types  ordered  unnamed  without resorting to non-supported techniques  Comparison to other answers The here proposed variants all consider trigger a compile error in case of duplicate or missing tuple items given a reference object type like IMyTable  For example declaring an array type of  keyof IMyTable    cannot catch these errors  In addition  they don t require a specific library  last variant uses ts-morph  which I would consider a generic compiler wrapper   emit a tuple type as opposed to an object  only first solution creates an array  or wide array type  compare to these answers  and lastly don t need classes  Variant 1  Simple typed array    Record type ensures  we have no double or missing keys  values can be neglected function createKeys keyRecord  Record lt keyof IMyTable  any gt     keyof IMyTable        return Object keys keyRecord  as any    const keys   createKeys   isDeleted  1  createdAt  1  title  1  id  1       const keys    quot id quot     quot title quot     quot createdAt quot     quot isDeleted quot        easiest  - manual with auto-completion - array  no tuple Playground If you don t like creating a record  take a look at this alternative with Set and assertion types   Variant 2  Tuple with helper function function createKeys lt T extends readonly  keyof IMyTable       keyof IMyTable  gt       t  T  amp  CheckMissing lt T  IMyTable gt   amp  CheckDuplicate lt T gt    T       return t      tuple  - manual with auto-completion  - more advanced  complex types Playground Explanation createKeys does compile-time checks by merging the function parameter type with additional assertion types  that emit an error for not suitable input   keyof IMyTable       keyof IMyTable  is a  quot black magic quot  way to force inference of a tuple instead of an array from the callee side  Alternatively  you can use const assertions   as const from caller side  CheckMissing checks  if T misses keys from U  type CheckMissing lt T extends readonly any    U extends Record lt string  any gt  gt           K in keyof U   K extends T number    never   K   keyof U  extends never   T   T  amp   quot Error  missing keys quot   type T1   CheckMissing lt   quot p1 quot     p1 any  p2 any  gt      quot p1 quot    amp   quot Error  missing keys quot  type T2   CheckMissing lt   quot p1 quot    quot p2 quot      p1  any  p2  any   gt       quot p1 quot    quot p2 quot    Note  T  amp   quot Error  missing keys quot  is just for nice IDE errors  You could also write never  CheckDuplicates checks double tuple items  type CheckDuplicate lt T extends readonly any   gt           P1 in keyof T    quot  flag  quot  extends        P2 in keyof T   P2 extends P1   never           T P2  extends T P1     quot  flag  quot    never   keyof T         T P1    quot Error  duplicate quot     T P1     type T3   CheckDuplicate lt  1  2  3  gt      1  2  3  type T4   CheckDuplicate lt  1  2  1  gt        1   quot Error  duplicate quot    2   1   quot Error  duplicate quot     Note  More infos on unique item checks in tuples are in this post  With TS 4 1  we also can name missing keys in the error string - take a look at this Playground   Variant 3  Recursive type With version 4 1  TypeScript officially supports conditional recursive types  which can be potentially used here as well  Though  the type computation is expensive due to combinatory complexity - performance degrades massively for more than 5-6 items  I list this alternative for completeness  Playground   type Prepend lt T  U extends any   gt     T     U     TS 4 0 variadic tuples  type Keys lt T extends Record lt string  any gt  gt    Keys  lt T     gt  type Keys  lt T extends Record lt string  any gt   U extends PropertyKey   gt             P in keyof T      extends Omit lt T  P gt     P    Prepend lt P  Keys  lt Omit lt T  P gt   U gt  gt      keyof T   const t1  Keys lt IMyTable gt      quot createdAt quot    quot isDeleted quot    quot id quot    quot title quot           tuple  - manual with auto-completion   no helper function -- performance  Variant 4  Code generator   TS compiler API ts-morph is chosen here  as it is a tad simpler wrapper alternative to the original TS compiler API  Of course  you can also use the compiler API directly  Let s look at the generator code       src mybuildstep ts import  Project  VariableDeclarationKind  InterfaceDeclaration   from  quot ts-morph quot    const project   new Project       source file with IMyTable interface const sourceFile   project addSourceFileAtPath  quot   src IMyTable ts quot        target file to write the keys string array to const destFile   project createSourceFile  quot   src generated IMyTable-keys ts quot    quot  quot       overwrite  true    overwrite if exists       function createKeys node  InterfaceDeclaration      const allKeys   node getProperties   map p   gt  p getName       destFile addVariableStatement       declarationKind  VariableDeclarationKind Const      declarations             name   quot keys quot           initializer  writer   gt            writer write    JSON stringify allKeys   as const                   createKeys sourceFile getInterface  quot IMyTable quot      destFile saveSync       flush all changes and write to disk  After we compile and run this file with tsc  amp  amp  node dist mybuildstep js  a file   src generated IMyTable-keys ts with following content is generated       src generated IMyTable-keys ts const keys     quot id quot   quot title quot   quot createdAt quot   quot isDeleted quot   as const     auto-generating solution   scalable for multiple properties   no helper function   tuple - extra build-step - needs familiarity with compiler API

[javascript] Get keys of a Typescript interface as array of strings

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