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Small projects scream dynamic types, but not every baby never grows up. Eventually the cognitive load becomes crippling and you're crying for static types.

Dynamic languages definitely help with the rapid development of a small application or prototype, but when you have a large team trying to maintain the application, static typing allows for better compile time error catching. Weird hybrid approaches makes me wary.

Facebook did even more with PHP with HHVM and other tooling. I'm not sure if JS was exactly the same for Microsoft as I saw it more as a cool internal project that got big vs something that eventually became necessary.

From what I read, several big teams at Microsoft were dogfooding Typescript immediately during development, even before it was ever externally released.