Definitions are things that you have given a name to: constants, and functions. Derw calls constants const. Each definition of is composed of two parts: a type line, specifying the type of the definition, and the body, specifying the value it should have.
A constant is simply a value that does not change.
A function takes arguments to produce a value.
"I don't know you"
Lambdas (or anonoymous functions) are functions without a name. You'd typically want to use them for making small functions that you pass to another function, like List.map.
Definitions within definitions
Sometimes, you'll want to define a function or a const inside another. This is particularly useful when you have a helper function that you wish to use with a set of arguments from the parent function. These can be done via let..in. let..in are converted to local variables within the parent code, so you don't need to worry about naming collisions. You can also use let..in in if and case branches.
In a function
In a const
Consts simply become TypeScript consts
Functions become TypeScript functions. All functions must have a return value
return"I don't know you";
Lambdas are turned into anonymous functions in TypeScript, with each argument being marked as any.