打字稿：类型交集和类型别名出现问题

Question

Using Typescript 2.1.4 , I bumped into some weird compilation problem while using intersection types ( type1 & type2 ) and type alias ( type Something<T> = ... ).

To explain what I tried to achieve: I simply wanted to declare a type alias which represents the intersection of some defined object values (in this case the property id of type string ) and of custom additional properties, all of them being optional. This type uses the Typescript pre-defined Partial type alias.

The following code compiles and works:

export type StoreModelPartial<T> = {id:string} & T;

function test<T>(obj:StoreModelPartial<T>) {
    console.log('TEST', obj);
}

console.log(test({
    'id':'value',
    'something':'value2'
}));

But when I try to use the Partial type alias, I get a compilation error:

export type StoreModelPartial<T> = Partial<{id:string} & T>;

function test<T>(obj:StoreModelPartial<T>) {
    console.log('TEST', obj);
}

console.log(test({
    'id':'value',
    'something':'value2'
}));

I get this error:

Argument of type '{ 'id': string; 'something': string; }' is not assignable to parameter of type 'Partial<{ id: string; } & T>

Any idea?

Answer 1

Ah forget it, I think I figured it out by writing this question... And re-reading the error message of my code sample, which is clearer than the more complex part I was fighting with originally.

I had thought intersection and extension were somewhat the same, but nope.

What I need is extends :

export type StoreModelPartial<T extends {id?:string}> = Partial<T>;

function test<T>(obj:StoreModelPartial<T>) {
    console.log('TEST', obj);
}

console.log(test({
    'id':'value',
    'something':'value2'
}));