How do you define a variable with a specific type in TypeScript?

Defining variables with specific types in TypeScript is straightforward. You can specify a variable's type by appending a colon (:) followed by the type name after the variable name. This enables TypeScript to enforce type safety by catching potential type errors during compilation.

Example

Suppose we want to define a variable representing a user's age, which we know should be a number. In TypeScript, you can define it as:

typescript
let age: number;
age = 30; // Correct

Here, the age variable is explicitly declared as a number type, meaning that if you attempt to assign a non-numeric value to age, the TypeScript compiler will report an error:

typescript
age = "thirty"; // Error: Type 'string' is not assignable to type 'number'.

More Complex Types

TypeScript also supports more complex type definitions, such as arrays, objects, and tuples. For example, if you want to define an array containing only strings, you can do:

typescript
let fruits: string[] = ["apple", "banana", "cherry"];

If you want to define an object, you can specify the types of its properties:

typescript
let person: { name: string; age: number } = {
    name: "Alice",
    age: 25
};

Using Interfaces or Type Aliases

For more complex data structures, you can define types using interfaces (interface) or type aliases (type), which simplify type reuse and extension.

typescript
interface User {
    name: string;
    age: number;
}

let user: User = {
    name: "Bob",
    age: 30
};

In this example, we define a User interface and use it to declare the type of the user variable.

Summary

By explicitly defining variable types in TypeScript, you can leverage its type system to improve code maintainability and reduce runtime errors. These type definitions can be simple primitive types or complex interfaces or type aliases, depending on your specific needs. This type safety feature is one of TypeScript's main advantages.