TypeScript allows strongly typed functions/lambda arguments.
For instance, you might define a logging routine, where you delegate the actual log writing (console output, file output, etc) to a callback.
To do this, you’d want to accept a function that takes a string, and returns nothing (unit, void, etc, depending on what you compare).
You can do this easily:
let onLog: (value: string)=>void =
console.log;