Comment by potatoz2

It just means that structural types are the default and you need to opt in to nominal types, when necessary: https://www.typescriptlang.org/play/?ssl=13&ssc=1&pln=13&pc=... (there are other ways as well)

In something like Java, it's the opposite: you get nominal types by default and you need to opt in to structural types (via interfaces).

Because TS is built on a duck typed language, tons of existing libraries would break if they had to declare explicit nominal types as arguments. Or you'd need to add custom interfaces for each library you use. Structural typing is what you want 99% of the time.