`Exact` type doesn't protect against errors

After this issue https://github.com/dotansimha/graphql-code-generator/issues/4120 in generated files appears Exact type

type Exact<T extends { [key: string]: unknown }> = {
  [K in keyof T]: T[K];
};

that doesn’t do anything

Example 1:

type Variables = { [key: string]: never }

declare const query: (variables: QueryVariables) => void

query({}) // Ok
query({a: 1}) // Argument of type '{ a: number; }' is not assignable to parameter of type 'Variables'

vs Exact version

type Variables = { [key: string]: never }

declare const query: (variables: Exact<Variables>) => void

query({}) // Ok
query({a: 1}) // Argument of type '{ a: number; }' is not assignable to parameter of type 'Variables'

It works absolutely the same

Example 2:

type Variables = { email: string }

declare const query: (variables: Variables) => void

query({email: 'email', password: 'password'}) // Argument of type '{ email: string; password: string; }' is not assignable to parameter of type 'Variables'.

const variables = {email: 'email', password: 'password'}
query(variables ) // Ok

I can pass additional fields and have no errors in TS, but have errors in runtime vs Exact version

type Variables = { email: string }

declare const query: (variables: Exact<Variables>) => void

query({email: 'email', password: 'password'}) // Argument of type '{ email: string; password: string; }' is not assignable to parameter of type 'Variables'.

const variables = {email: 'email', password: 'password'}
query(variables ) // Ok

It works absolutely the same and doesn’t protect against errors

Link to the TS playground

[charlypoly]

Hi @oceandrama,

Yes, it seems that the Exact<T> type helper is kind of a noop.

After doing some research, I found this famous implementation that does the work: https://fettblog.eu/typescript-match-the-exact-object-shape/

type ValidateStructure<T, Struct> = 
  T extends Struct ? 
  Exclude<keyof T, keyof Struct> extends never ? T : never : never;

Playground example from the article

This is the solution mentioned many times in the official TypeScript issue for “Exact Types”: https://github.com/microsoft/TypeScript/issues/12936

I guess we could migrate from Exact<T> to ValidateStructure<T, Struct> but it would required a lot of work because ValidateStructure<T, Struct> requires Generic argument parameters while Exact<T> did not

[tv42]

Duplicate of #4577.