Interfaces can be augmented. What does it mean: if you export an interface, someone who imports it can literally add members to it without modifying your code. Why? Mainly for globals like Window (interface of window): you import it, augment, and further have typesafe global in your whole app. And this is done without modifying library which exports Window for your.
As a consequence, much of TypeScript metaprogramming does not work with interfaces — because they can be augmented, there's little sense in keyof Window (result can be anything), or in trying to create mapped interfaces (only mapped types possible).
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Interfaces can be augmented. What does it mean: if you export an interface, someone who imports it can literally add members to it without modifying your code. Why? Mainly for globals like
Window(interface ofwindow): you import it, augment, and further have typesafe global in your whole app. And this is done without modifying library which exportsWindowfor your.As a consequence, much of TypeScript metaprogramming does not work with interfaces — because they can be augmented, there's little sense in
keyof Window(result can be anything), or in trying to create mapped interfaces (only mapped types possible).