Enforcing the type of the indexed members of a Typescript object

Question

I would like to store a mapping of string -  string in a Typescript object  and enforce that all of the keys map to strings   For example   var stuff       stuff  a      foo        okay stuff  b      bar        okay stuff  c     false       ERROR   bool    string   Is there a way for me to enforce that the values must be strings  or whatever type

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Define interface  interface Settings     lang   en     da     welcome  boolean      Enforce key to be a specific key of Settings interface  private setSettings key  keyof Settings  value  any          Update settings key

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interface AgeMap        name  string   number    const friendsAges  AgeMap          Sandy   34       Joe   28       Sarah   30       Michelle    fifty      ERROR  Type  string  is not assignable to type  number        Here  the interface AgeMap enforces keys as strings  and values as numbers   The keyword name can be any identifier and should be used to suggest the syntax of your interface type   You can use a similar syntax to enforce that an object has a key for every entry in a union type   type DayOfTheWeek    sunday     monday     tuesday     wednesday     thursday     friday     saturday    type ChoresMap      day in DayOfTheWeek   string     const chores  ChoresMap        ERROR  Property  saturday  is missing in type            sunday    do the dishes        monday    walk the dog        tuesday    water the plants        wednesday    take out the trash        thursday    clean your room        friday    mow the lawn        You can  of course  make this a generic type as well   type DayOfTheWeek    sunday     monday     tuesday     wednesday     thursday     friday     saturday    type DayOfTheWeekMap lt T gt       day in DayOfTheWeek   T     const chores  DayOfTheWeekMap lt string gt           sunday    do the dishes        monday    walk the dog        tuesday    water the plants        wednesday    take out the trash        thursday    clean your room        friday    mow the lawn        saturday    relax       const workDays  DayOfTheWeekMap lt boolean gt           sunday   false       monday   true       tuesday   true       wednesday   true       thursday   true       friday   true       saturday   false       10 10 2018 update  Check out  dracstaxi s answer below - there s now a built-in type Record which does most of this for you   1 2 2020 update  I ve entirely removed the pre-made mapping interfaces from my answer    dracstaxi s answer makes them totally irrelevant   If you d still like to use them  check the edit history

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A quick update  since Typescript 2 1 there is a built in type Record lt T  K gt  that acts like a dictionary  In this case you could declare stuff like so  var stuff  Record lt string  any gt         You could also limit specify potential keys by unioning literal types  var stuff  Record lt  a   b   c   string boolean gt         Here s a more generic example using the record type from the docs     For every properties K of type T  transform it to U function mapObject lt K extends string  T  U gt  obj  Record lt K  T gt   f   x  T    gt  U   Record lt K  U gt   const names     foo   quot hello quot   bar   quot world quot   baz   quot bye quot     const lengths   mapObject names  s   gt  s length         foo  number  bar  number  baz  number    TypeScript 2 1 Documentation on Record lt T  K gt  The only disadvantage I see to using this over   key  T   K  is that you can encode useful info on what sort of key you are using in place of  quot key quot  e g  if your object only had prime keys you could hint at that like so    prime  number   yourType   Here s a regex I wrote to help with these conversions  This will only convert cases where the label is  quot key quot   To convert other labels simply change the first capturing group  Find     s    key  s    s   s   w   s    s   s           s    s    Replace  Record lt  2   3 gt

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You can pass a name to the unknown key and then write your types  type StuffBody        key  string   string      Now you can use it in your type checking  let stuff  StuffBody        But for FlowType there is no need to have name  type StuffBody        string   string

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var stuff     key  string   string          stuff  a            ok stuff  a     4      error         or  if you re using this a lot and don t want to type so much     interface StringMap    key  string   string    var stuff2  StringMap           same as above

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Building on  shabunc s answer  this would allow enforcing either the key or the value     or both     to be anything you want to enforce   type IdentifierKeys    my valid key 1     my valid key 2   type IdentifierValues    my valid value 1     my valid value 2    let stuff   new Map lt IdentifierKeys  IdentifierValues gt       Should also work using enum instead of a type definition

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Ryan Cavanaugh s answer is totally ok and still valid  Still it worth to add that as of Fall 16 when we can claim that ES6 is supported by the majority of platforms it almost always better to stick to Map whenever you need associate some data with some key    When we write let a     s  string   string    we need to remember that after typescript compiled there s not such thing like type data  it s only used for compiling  And    s  string   string    will compile to just       That said  even if you ll write something like   class TrickyKey      let dict    key TrickyKey   string         This just won t compile  even for target es6  you ll get error TS1023  An index signature parameter type must be  string  or  number    So practically you are limited with string or number as potential key so there s not that much of a sense of enforcing type check here  especially keeping in mind that when js tries to access key by number it converts it to string   So it is quite safe to assume that best practice is to use Map even if keys are string  so I d stick with   let staff  Map lt string  string gt    new Map

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interface AccountSelectParams           const params             const tmpParams     key in keyof AccountSelectParams   any     undefined         for  const key of Object keys params         const customKey    key as keyof typeof params       if  key in params  amp  amp  params customKey   amp  amp   this state customKey           tmpParams customKey    params customKey              please commented if you get the idea of this concept

[typescript] Enforcing the type of the indexed members of a Typescript object?

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