What is the reason why the code does not fail when executing a = b ? Check out this link
type A = {
x?: number
y?: number
}
type B = {
x?: number
z: number
}
let a: A = {}
let b: B = {z: 1}
a = b // does not fail
a = {z: 1} // fails
Typescript uses structural typing (Wikipedia), where an object of type B is considered a valid A object.
In the second scenario, excess property checks will prevent you from assigning a literal that shares no similarities with A.
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