Type annotation on variables in Typescript

Introduction

Type annotation helps you tell the computer what kind of value a variable will hold. This makes your code safer and easier to understand.

When you want to make sure a variable only holds numbers.

When you want to explain to others what type of data a variable should have.

When you want to catch mistakes early if you try to put the wrong type of value in a variable.

When you want your code editor to give you helpful hints and warnings.

When you are working on bigger projects with many people and want clear rules for data.

Syntax

Typescript

let variableName: type = value;

The colon : is used to add the type after the variable name.

You can use many types like number, string, boolean, or even custom types.

Examples

This means age can only hold numbers.

Typescript

let age: number = 25;

This means name can only hold text (strings).

Typescript

let name: string = "Alice";

This means isStudent can only be true or false.

Typescript

let isStudent: boolean = true;

You can declare a variable with a type but assign a value later.

Typescript

let score: number;

Sample Program

This program shows how to use type annotations for different variables and prints their values.

Typescript

let height: number = 180;
let firstName: string = "John";
let isActive: boolean = false;

console.log(`Name: ${firstName}`);
console.log(`Height: ${height} cm`);
console.log(`Active user: ${isActive}`);

Important Notes

If you try to assign a value of the wrong type, TypeScript will show an error before running your code.

Type annotations are optional if TypeScript can guess the type from the value, but adding them helps clarity.

Summary

Type annotations tell what kind of value a variable can hold.

They help catch mistakes early and make code easier to read.

Use a colon : after the variable name to add a type.