No Error in Typescript if spreading unknon property into variable

Question

I have a playground for this question here.

In the following code, typescript does error on the assignment to t2_2 ("not assignable") but not on t2_1, where I have the "x" property in some other variable, which I spread into the assignment.

Why does it not complain about the assignment to t2_1?

type Type1 {
    a: number;
    b: number;
}

type Type2 = {
    t1: Type1
    s: string;
}

let t1: Type1 = {a:1,b:2}
let tx = {x:2};

let t2_1: Type2 = {
    t1,
    s: "asdf",
    ...tx
}

let t2_2: Type2 = {
    t1,
    s: "asdf",
    x: 3
}

Answer 1

When you spread in tx, you don't know what properties it really has. So TypeScript doesn't really know if you have excess properties in some cases.

However, x very explicitly appears to be an excess property. It is statically known to be there - TS can always tell.

See

• https://github.com/microsoft/TypeScript/issues/14853
• https://github.com/microsoft/TypeScript/issues/12717
• https://github.com/microsoft/TypeScript/issues/2103#issuecomment-265513297