Structural typing
In TypeScript, usually, if things quack like ducks, then TypeScript assumes that they're ducks. Duck typing as it is called is baked into the language. We can say that TypeScript considers the compatibility of types. We’ll leverage this great feature in the projects to make things clear.
To give you an idea, if some function expects to receive an object of a Foo type and is instead called with an object of a Bar type, then TypeScript will not complain as long as Bar has the same structure as Foo. That is to say, a Bar object can be considered to be Foo if it exposes at least the expected properties/methods.
This is an oversimplification as there are actually many rules at play, depending on the considered types.
Structural typing is very useful as it helps avoid writing quite some boilerplate code; one example is when you have two equivalent data structures. In programming languages that don't support structural typing, you have to manually convert from one type to the other, just because the compiler doesn't recognize the structural compatibility of the types.
TypeScript does also supports some nominal typing (that is, non-structural). Nominal typing is interesting for cases where you want to distinguish objects with different types, even if their structure is identical.
We won't be covering nominal typing specifically in this book as it is more advanced, but do check out the following link if you're curious: https://basarat.gitbooks.io/typescript/docs/tips/nominalTyping.html.