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explanation on behaviour of `keyof` operator on generic types

1

I've stumbled across a small issue, which I actually managed to solve. However, I am looking for an explanation why this is an issue in the first place.

Lets say we have the following types:

interface Base 
 name: string;


interface TypeA extends Base 
 foo: number;
 

interface TypeB extends Base 
 bar: string;

And we have the following generic function:

const genericSet = <T>(obj: T, partial: Partial<T>) => ( ...obj, ...partial );
const demo = genericSet( b: '222', a: 111 , a: 333 );
/* works great */

but now let's call the function from within another generic function:

function doSomething<T extends Base>(obj: T) 
 const newObj = genericSet(obj, name: 'a' ); //error!, argument name: string isn't assignable to Partial<T>
 ...

But if we define the function like this, basically just rewriting Partial

const genericSet2 = <T, K extends keyof T>(obj: T, partial:[key in K]?: T[key] ) => ( ...obj, ...partial );

Than everything works!

I'm just looking for information why. Intuitively I can guess it's something related when typescript actually computes types, but I would like a concrete explanation on why it works (If nothing else, just so I'm familiar with actual terms)

If you know of any relevant github issues/commits, it would also be great.

typescript generics

share|improve this question

asked Mar 7 at 14:01

Aviad HadadAviad Hadad

90449

add a comment | 

1

I've stumbled across a small issue, which I actually managed to solve. However, I am looking for an explanation why this is an issue in the first place.

Lets say we have the following types:

interface Base 
 name: string;


interface TypeA extends Base 
 foo: number;
 

interface TypeB extends Base 
 bar: string;

And we have the following generic function:

const genericSet = <T>(obj: T, partial: Partial<T>) => ( ...obj, ...partial );
const demo = genericSet( b: '222', a: 111 , a: 333 );
/* works great */

but now let's call the function from within another generic function:

function doSomething<T extends Base>(obj: T) 
 const newObj = genericSet(obj, name: 'a' ); //error!, argument name: string isn't assignable to Partial<T>
 ...

But if we define the function like this, basically just rewriting Partial

const genericSet2 = <T, K extends keyof T>(obj: T, partial:[key in K]?: T[key] ) => ( ...obj, ...partial );

Than everything works!

I'm just looking for information why. Intuitively I can guess it's something related when typescript actually computes types, but I would like a concrete explanation on why it works (If nothing else, just so I'm familiar with actual terms)

If you know of any relevant github issues/commits, it would also be great.

typescript generics

share|improve this question

asked Mar 7 at 14:01

Aviad HadadAviad Hadad

90449

add a comment | 

I've stumbled across a small issue, which I actually managed to solve. However, I am looking for an explanation why this is an issue in the first place.

Lets say we have the following types:

interface Base 
 name: string;


interface TypeA extends Base 
 foo: number;
 

interface TypeB extends Base 
 bar: string;

And we have the following generic function:

const genericSet = <T>(obj: T, partial: Partial<T>) => ( ...obj, ...partial );
const demo = genericSet( b: '222', a: 111 , a: 333 );
/* works great */

but now let's call the function from within another generic function:

function doSomething<T extends Base>(obj: T) 
 const newObj = genericSet(obj, name: 'a' ); //error!, argument name: string isn't assignable to Partial<T>
 ...

But if we define the function like this, basically just rewriting Partial

const genericSet2 = <T, K extends keyof T>(obj: T, partial:[key in K]?: T[key] ) => ( ...obj, ...partial );

Than everything works!

I'm just looking for information why. Intuitively I can guess it's something related when typescript actually computes types, but I would like a concrete explanation on why it works (If nothing else, just so I'm familiar with actual terms)

If you know of any relevant github issues/commits, it would also be great.

typescript generics

share|improve this question

asked Mar 7 at 14:01

Aviad HadadAviad Hadad

90449

interface Base 
 name: string;


interface TypeA extends Base 
 foo: number;
 

interface TypeB extends Base 
 bar: string;

const genericSet = <T>(obj: T, partial: Partial<T>) => ( ...obj, ...partial );
const demo = genericSet( b: '222', a: 111 , a: 333 );
/* works great */

function doSomething<T extends Base>(obj: T) 
 const newObj = genericSet(obj, name: 'a' ); //error!, argument name: string isn't assignable to Partial<T>
 ...

const genericSet2 = <T, K extends keyof T>(obj: T, partial:[key in K]?: T[key] ) => ( ...obj, ...partial );

share|improve this question

asked Mar 7 at 14:01

Aviad HadadAviad Hadad

90449

share|improve this question

asked Mar 7 at 14:01

Aviad HadadAviad Hadad

90449

asked Mar 7 at 14:01

Aviad HadadAviad Hadad

90449

asked Mar 7 at 14:01

Aviad HadadAviad Hadad

90449