Variables in Typescript have a type (not in Javascript), so we need a way to indicate the type of a variable:

let a: number;
let b = 3; // inferred type: number

function sum(a: number, b: number) {
  return a + b;
} // return type inferred: number

Typescript uses very powerful type inference, so if it can infer the type from the context, we can omit the annotation and the result is exactly the same.

Type annotations are what distinguishes Typescript from Javascript so when you compile a Typescript file to Javascript, it usually looks as if the Typescript source was stripped of all the type information.

Any

There is a special type called any which indicates that a variable can be of any type (the default in Javascript). Whenever you are dumbfounded by Typescript errors, it can be a way to escape the type system temporarily. However, in the long run, this is a bad idea.

let b: any = 1;
b = 'one and the same';