Why is the Keyword `as` Needed When Narrowing TypeScript Interfaces With an `if`?

Given some interfaces for Circle and Square as follows:

export default interface Circle { radius: number } 

export default interface Square { sideLength: number; } 

I can use as to calculate the area properly:

function getArea(shape: Circle | Square) { if ((shape as Circle).radius) { return Math.PI * (shape as

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