What is the detail logic of type assertion?

[chinesedfan]

TypeScript Version: 2.8.1

Search Terms: type assertion

Code

interface PM_MGEInfo {
    category: string;
    bid: string;
    cid?: string;
    labs?: { [key: string]: any };
}

function makeMgeInfo1(bid: string): PM_MGEInfo {
    return <PM_MGEInfo>{
        bid
    };
}

function makeMgeInfo2(bid: string): PM_MGEInfo {
    return <PM_MGEInfo>{
        bid,
        labs: {}
    };
}

function makeMgeInfo3(bid: string): PM_MGEInfo {
    return <PM_MGEInfo>{
        bid,
        // error TS2352: Type '{ labs: { poi_id: string; }; bid: string; }' cannot be converted to type 'PM_MGEInfo'.
        // Property 'category' is missing in type '{ labs: { poi_id: string; }; bid: string; }'.
        labs: {a: 1}
    };
}

Expected behavior:
The compiler always complains the missing of field category, or never. I know type assertion is not 100% safe, but am confused about its check logic. Can someone point out related codes for me? The ts code base is so complex.

Actual behavior:
Only makeMgeInfo3 throws an error about the missing of field category.

Playground Link: very long link

Related Issues:
I also asked on stackoverflow, but got no satisfying answers.

[mhegazy]

Docs are at: https://www.typescriptlang.org/docs/handbook/basic-types.html#type-assertions

Basically the compiler is checking that the two types are somehow related, that is that one is assignable to the other. In this case neither is the case. In one way the object literal is missing a a required property category, in the other way missing required property labs.a

[chinesedfan]

In this case neither is the case. In one way the object literal is missing a a required property category, in the other way missing required property labs.a

@mhegazy Can you explain a little more about why case 1&2 are no compilation errors? What's the difference between 1/2 and 3?

[mhegazy]

Cuase they are assignable in one direction.

[chinesedfan]

@mhegazy Sorry, I still don't understand. I think the 3 object literals are not assignable to PM_MGEInfo, due to the lack of property category. But why does only the last case trigger the check?

If it is hard to explain, can you give me a link to the related TypeScript source codes?

[DanielRosenwasser]

There's a function in src/compiler/checker.ts called checkAssertion. I can't link it here because the file's too big.

[typescript-bot]

Automatically closing this issue for housekeeping purposes. The issue labels indicate that it is unactionable at the moment or has already been addressed.