Typescript - Assign object with extra property do not fail

Question

Why this code do not fail when execute a = b ? Play link

type A = {
    x?: number
    y?: number
}

type B = {
    x?: number
    z: number
}

let a: A = {}
let b: B = {z: 1}

a = b // not fail
a = {z: 1} // fail

https://www.typescriptlang.org/docs/handbook/interfaces.html#excess-property-checks — VLAZ, Aug 23 '22 at 13:07

score 1 · Accepted Answer · edited Aug 23 '22 at 13:39

1

Typescript uses structural typing (Wikipedia), an object of type B is a valid A object.

In your second example, excess property checks will prevent you to assign a literal that has nothing in common with A.

edited Aug 23 '22 at 13:39

DerStoffel

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answered Aug 23 '22 at 13:02

Matthieu Riegler

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Typescript - Assign object with extra property do not fail

1 Answers1