4. Type Annotations on Variables

When you declare a variable using const, var, or let, you can optionally add a type annotation to explicitly specify the type of the variable:

let myName: string = "Alice";

TypeScript doesn't use “types on the left”-style declarations like int x = 0; Type annotations will always go after the thing being typed.

In most cases, though, this isn't needed. Wherever possible, TypeScript tries to automatically infer the types in your code. For example, the type of a variable is inferred based on the type of its initializer:

// No type annotation needed -- 'myName' inferred as type 'string'
let myName = "Alice";

For the most part you don't need to explicitly learn the rules of inference. If you're starting out, try using fewer type annotations than you think - you might be surprised how few you need for TypeScript to fully understand what's going on.

— The TypeScript Handbook: Everyday Types

Exercises