Typescript: issue with type intersection and type alias

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?

arvymetal · Accepted Answer · 2017-01-26 17:15:45Z

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'
}));

answered Jan 26, 2017 at 17:15

arvymetal

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