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TypeScript Annotations
TypeScript: Type Annotations
Type annotations let you tell TypeScript exactly what kind of data a variable or function should use.
What Are Type Annotations?
Sometimes, you want to be very clear about what kind of value you're expecting.
Type annotations let you label a variable or a function with its expected type.
It’s like giving TypeScript instructions:
“This should always be a number” or “This function must return a string.”
How to Write One
You add a colon : after the variable name, followed by the type.
let username: string = "Sahand"; let age: number = 30; Let isOnline: boolean = true;
TypeScript will now keep an eye on those variables to make sure their types never change.
Function Annotations
You can also use annotations in functions — for both parameters and return values.
function greet(name: string): string {
return "Hello, " + name;
}
- name: string means the function expects a string as input.
- : string after the parentheses means the function returns a string.
Why Use Them?
- Makes your code easier to read
- Helps catch bugs early
- Great for teams or large codebases
- Works well when TypeScript can't figure out the type on its own
Tip
Use annotations when:
- Variables have no initial value
- Parameters and return types need to be clear
- Working with complex data structures (like objects or functions)
Prefer Learning by Watching?
Watch these YouTube tutorials to understand TYPESCRIPT Tutorial visually:
What You'll Learn:
- 📌 Type Annotations - TypeScript Programming Tutorial #1
- 📌 Type Annotation & Type Inference | String Templates | TypeScript Tutorial