Declare and initialize a Dictionary in Typescript

Question

Given the following code  interface IPerson      firstName  string     lastName  string     var persons     id  string   IPerson            p1     firstName   F1   lastName   L1         p2     firstName   F2         Why isn t the initialization rejected  After all  the second object does not have the  lastName  property

User · Answer

Typescript fails in your case because it expects all the fields to be present. Use Record and Partial utility types to solve it.

Record<string, Partial<IPerson>>

interface IPerson {
   firstName: string;
   lastName: string;
}

var persons: Record<string, Partial<IPerson>> = {
   "p1": { firstName: "F1", lastName: "L1" },
   "p2": { firstName: "F2" }
};

Explanation.

Record type creates a dictionary/hashmap.
Partial type says some of the fields may be missing.

Alternate.

If you wish to make last name optional you can append a ? Typescript will know that it's optional.

lastName?: string;

https://www.typescriptlang.org/docs/handbook/utility-types.html

User · Answer

Edit  This has since been fixed in the latest TS versions  Quoting  Simon Weaver s comment on the OP s post      Note  this has since been fixed  not sure which exact TS version   I   get these errors in VS  as you would expect  Index signatures are   incompatible  Type    firstName  string     is not assignable to type    IPerson   Property  lastName  is missing in type    firstName    string         Apparently this doesn t work when passing the initial data at declaration  I guess this is a bug in TypeScript  so you should raise one at the project site   You can make use of the typed dictionary by splitting your example up in declaration and initialization  like   var persons     id  string    IPerson          persons  p1       firstName   F1   lastName   L1     persons  p2       firstName   F2        will result in an error

User · Answer

Here is a more general Dictionary implementation inspired by this from  dmck        interface IDictionary lt T gt          add key  string  value  T   void        remove key  string   void        containsKey key  string   boolean        keys    string          values    T               class Dictionary lt T gt  implements IDictionary lt T gt            keys  string                values  T                constructor init     key  string  value  T                 if  init              for  var x   0  x  lt  init length  x                  this init x  key    init x  value              this  keys push init x  key               this  values push init x  value                                        add key  string  value  T            this key    value          this  keys push key           this  values push value                  remove key  string            var index   this  keys indexOf key  0           this  keys splice index  1           this  values splice index  1            delete this key                  keys    string             return this  keys                 values    T             return this  values                 containsKey key  string            if  typeof this key       undefined               return false                     return true                 toLookup    IDictionary lt T gt            return this

User · Answer

I agree with thomaux that the initialization type checking error is a TypeScript bug  However  I still wanted to find a way to declare and initialize a Dictionary in a single statement with correct type checking  This implementation is longer  however it adds additional functionality such as a containsKey key  string  and remove key  string  method  I suspect that this could be simplified once generics are available in the 0 9 release   First we declare the base Dictionary class and Interface  The interface is required for the indexer because classes cannot implement them   interface IDictionary       add key  string  value  any   void      remove key  string   void      containsKey key  string   bool      keys    string        values    any       class Dictionary         keys  string     new string         values  any     new any         constructor init    key  string  value  any                  for  var x   0  x  lt  init length  x                  this init x  key    init x  value              this  keys push init x  key               this  values push init x  value                        add key  string  value  any            this key    value          this  keys push key           this  values push value              remove key  string            var index   this  keys indexOf key  0           this  keys splice index  1           this  values splice index  1            delete this key              keys    string             return this  keys             values    any             return this  values             containsKey key  string            if  typeof this key       undefined                 return false                     return true             toLookup    IDictionary           return this            Now we declare the Person specific type and Dictionary Dictionary interface  In the PersonDictionary note how we override values   and toLookup   to return the correct types   interface IPerson       firstName  string      lastName  string     interface IPersonDictionary extends IDictionary        index  string   IPerson      values    IPerson       class PersonDictionary extends Dictionary       constructor init    key  string  value  IPerson                 super init              values    IPerson            return this  values             toLookup    IPersonDictionary           return this            And here is a simple initialization and usage example   var persons   new PersonDictionary         key   p1   value    firstName   F1   lastName   L2             key   p2   value    firstName   F2   lastName   L2             key   p3   value    firstName   F3   lastName   L3         toLookup      alert persons  p1   firstName         persons  p1   lastName      alert  F1 L2  persons remove  p2     if   persons containsKey  p2          alert  Key no longer exists           alert  Key no longer exists    alert persons keys   join            alert  p1  p3

User · Answer

If you want to ignore a property  mark it as optional by adding a question mark   interface IPerson       firstName  string      lastName   string

User · Answer

For using dictionary object in typescript you can use interface as below   interface Dictionary lt T gt         Key  string   T      and  use this for your class property type   export class SearchParameters       SearchFor  Dictionary lt string gt            to use and initialize this class   getUsers    Observable lt any gt            var searchParams   new SearchParameters            searchParams SearchFor  userId      1           searchParams SearchFor  userName      xyz            return this http post searchParams   users search                map res   gt                    return res                              catch this handleError bind this

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