It's also considered by many to be an antipattern to add a type that is otherwise inferred. For a trivial example,

let x: number = 6;

You should instead allow the compiler to infer where it can and do

let x = 6;

Inference doesn't work across function calls like it does (did?) in Flow, which is a good thing, so those always need to be typed.

If you wanted the type of x to be 6 instead of number, you would use

let x = 6 as const;