Overriding interface property type defined in Typescript d ts file

Question

Is there a way to change the type of interface property defined in a   d ts in typescript   for example  An interface in x d ts is defined as    interface A     property  number      I want to change it in the typescript files that I write to   interface A     property  Object      or even this would work  interface B extends A     property  Object      Will this approach work  It didn t work when I tried on my system  Just want to confirm if it s even possible

User · Accepted Answer

I use a method that first filters the fields and then combines them.

reference Exclude property from type

interface A {
    x: string
}

export type B = Omit<A, 'x'> & { x: number };

for interface:

interface A {
    x: string
}

interface B extends Omit<A, 'x'> {
  x: number
}

User · Answer

Omit the property when extending the interface  interface A     a  number    b  number     interface B extends Omit lt A   a  gt      a  boolean

User · Answer

type ModifiedType   Modify lt OriginalType      a  number    b  number    gt   interface ModifiedInterface extends Modify lt OriginalType      a  number    b  number    gt        Inspired by ZSkycat s extends Omit solution  I came up with this    type Modify lt T  R gt    Omit lt T  keyof R gt   amp  R      before typescript 3 5 type Modify lt T  R gt    Pick lt T  Exclude lt keyof T  keyof R gt  gt   amp  R    Example   interface OriginalInterface     a  string    b  boolean    c  number     type ModifiedType    Modify lt OriginalInterface       a  number    b  number    gt      ModifiedType     a  number  b  number  c  number      Going step by step   type R0   Omit lt OriginalType   a     b  gt              c  number    type R1   R0  amp   a  number  b  number                a  number  b  number  c  number     type T0   Exclude lt  a     b     c     a     b  gt      c  type T1   Pick lt OriginalType  T0 gt                     c  number    type T2   T1  amp   a  number  b  number                a  number  b  number  c  number      TypeScript Utility Types

User · Answer

You can t change the type of an existing property     You can add a property   interface A       newProperty  any      But changing a type of existing one   interface A       property  any      Results in an error      Subsequent variable declarations must have the same type   Variable    property  must be of type  number   but here has type  any    You can of course have your own interface which extends an existing one  In that case  you can override a type only to a compatible type  for example   interface A       x  string   number     interface B extends A       x  number        By the way  you probably should avoid using Object as a type  instead use the type any   In the docs for the any type it states      The any type is a powerful way to work with existing JavaScript    allowing you to gradually opt-in and opt-out of type-checking during   compilation  You might expect Object to play a similar role  as it   does in other languages  But variables of type Object only allow you   to assign any value to them - you can   t call arbitrary methods on   them  even ones that actually exist    let notSure  any   4  notSure ifItExists       okay  ifItExists might exist at runtime notSure toFixed       okay  toFixed exists  but the compiler doesn t check   let prettySure  Object   4  prettySure toFixed       Error  Property  toFixed  doesn t exist on type  Object

User · Answer

The short answer for lazy people like me  type Overrided   Omit lt YourInterface   overrideField  gt   amp    overrideField   lt type gt

User · Answer

It s funny I spend the day investigating possibility to solve the same case  I found that it not possible doing this way      a ts - module export interface A       x  string   any        b ts - module import  A  from    a    type SomeOtherType       coolStuff  number    interface B extends A       x  SomeOtherType      Cause A module may not know about all available types in your application  And it s quite boring port everything from everywhere and doing code like this    export interface A       x  A   B   C   D     Million Types Later     You have to define type later to have autocomplete works well      So you can cheat a bit      a ts - module export interface A       x  string      Left the some type by default  that allow autocomplete works  when overrides not required   Then     b ts - module import  A  from    a    type SomeOtherType       coolStuff  number        ts-ignore interface B extends A       x  SomeOtherType      Disable stupid exception here using  ts-ignore flag  saying us the we doing something wrong  And funny thing everything works as expected   In my case I m reducing the scope vision of type x  its allow me doing code more stricted  For example you have list of 100 properties  and you reduce it to 10  to avoid stupid situations

User · Answer

NOTE  Not sure if the syntax I m using in this answer was available when the older answers were written  but I think that this is a better approach on how to solve the example mentioned in this question   I ve had some issues related to this topic  overwriting interface properties   and this is how I m handling it   First create a generic interface  with the possible types you d like to use   You can even use choose a default value for the generic parameter as you can see in  lt T extends number   SOME OBJECT   number gt  type SOME OBJECT     foo   quot bar quot     interface INTERFACE A  lt T extends number   SOME OBJECT   number gt      property  T      Then you can create new types based on that contract  by passing a value to the generic parameter  or omit it and use the default    type A NUMBER   INTERFACE A                       USES THE default   number TYPE  SAME AS INTERFACE A lt number gt  type A SOME OBJECT   INTERFACE A lt SOME OBJECT gt      MAKES   property  SOME OBJECT    And this is the result  const aNumber  A NUMBER         property  111     THIS EXPECTS A NUMBER    const anObject  A SOME OBJECT         property         THIS EXPECTS SOME OBJECT         foo   quot bar quot           Typescript playground

User · Answer

I have created this type that allows me to easily override nested interfaces  export type DeepPartialAny lt T gt         P in keyof T    T P  extends Obj   DeepPartialAny lt T P  gt    any      export type Override lt A extends Obj  AOverride extends DeepPartialAny lt A gt  gt       K in keyof A     AOverride K  extends never       A K        AOverride K  extends Obj       Override lt A K   AOverride K  gt        AOverride K      And then you can use it like that  interface Foo     Bar        Baz  string         type Foo2   Override lt Foo    Bar    Baz  number     gt    const bar  Foo2  Bar    Baz     1     number

User · Answer

Extending  zSkycat s answer a little  you can create a generic that accepts two object types and returns a merged type with the members of the second overriding the members of the first   type Omit lt T  K extends keyof T gt    Pick lt T  Exclude lt keyof T  K gt  gt  type Merge lt M  N gt    Omit lt M  Extract lt keyof M  keyof N gt  gt   amp  N   interface A       name  string      color   string        redefine name to be string   number type B   Merge lt A        name  string   number      favorite   boolean    gt    let one  A         name   asdf       color   blue         A can become B because the types are all compatible let two  B   one   let three  B         name  1     three name    Bee   three favorite   true  three color    green       B cannot become A because the type of name  string   number  isn t compatible    with A even though the value is a string    Error  Type       is not assignable to type A let four  A   three

User · Answer

If someone else needs a generic utility type to do this  I came up with the following solution          Returns object T  but with T K  overridden to type U      example    type MyObject     a  number  b  string      OverrideProperty lt MyObject   a   string gt     returns   a  string  b  string       export type OverrideProperty lt T  K extends keyof T  U gt    Omit lt T  K gt   amp     P in keyof Pick lt T  K gt    U      I needed this because in my case  the key to override was a generic itself   If you don t have Omit ready  see Exclude property from type

User · Answer

For narrowing the type of the property  simple extend works perfect  as in Nitzan s answer  interface A       x  string   number     interface B extends A       x  number      For widening  or generally overriding the type  you can do Zskycat s solution  interface A       x  string    export type B   Omit lt A   x  gt   amp    x  number     But  if your interface A is extending a general interface  you will lose the custom types of A s remaining properties when using Omit  e g  interface A extends Record lt string   number  number   string   boolean gt        x  string      y  boolean     export type B   Omit lt A   x  gt   amp    x  number     let b  B     x  2  y   quot hi quot        no error on b y    The reason is  Omit internally only goes over Exclude lt keyof A   x  gt  keys which will be the general string   number in our case  So  B would become  x  number    and accepts any extra property with the type of number   string   boolean   To fix that  I came up with a different OverrideProps utility type as following  type OverrideProps lt M  N gt       P in keyof M   P extends keyof N   N P    M P      Example  type OverrideProps lt M  N gt       P in keyof M   P extends keyof N   N P    M P      interface A extends Record lt string   number  number   string   boolean gt        x  string      y  boolean     export type B   OverrideProps lt A    x  number   gt    let b  B     x  2  y   quot hi quot        error  b y should be boolean

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