TypeScript Variables

last modified March 3, 2025

Variables in TypeScript store data with strict type annotations. They enhance JavaScript variables by adding type safety and better tooling support. This tutorial explores variable declarations, type annotations, and best practices through practical examples.

Basic Variable Declarations

TypeScript variables can be declared using let, const, or var. This example shows basic variable declarations with types.

basic_variables.ts

let age: number = 30;
const name: string = "Alice";
var isActive: boolean = true;

console.log(age, name, isActive);  // Output: 30 Alice true

TypeScript enforces type annotations, ensuring variables hold the correct data types throughout their lifecycle.

Type Inference

TypeScript infers types when variables are initialized without explicit annotations. This reduces redundancy while maintaining type safety.

type_inference.ts

let score = 95;        // TypeScript infers `number`
const message = "Hi";  // TypeScript infers `string`

console.log(score, message);  // Output: 95 Hi

Type inference works for primitive types, arrays, and objects. Explicit annotations are recommended for complex types.

Union Types

Variables can hold multiple types using union types. This is useful for flexible data handling.

union_types.ts

let id: string | number;
id = "ABC123";
console.log(id);  // Output: ABC123

id = 123;
console.log(id);  // Output: 123

Union types allow variables to switch between specified types, providing flexibility while maintaining type safety.

Literal Types

Literal types restrict variables to specific values. This is useful for enumerations or fixed sets of values.

literal_types.ts

let status: "active" | "inactive";
status = "active";
console.log(status);  // Output: active

// status = "pending";  // Error: Type '"pending"' is not assignable

Literal types ensure variables only hold predefined values, reducing runtime errors.

Arrays and Tuples

Arrays and tuples store collections of values. TypeScript enforces element types for arrays and fixed structures for tuples.

arrays_tuples.ts

let numbers: number[] = [1, 2, 3];
let person: [string, number] = ["Alice", 30];

console.log(numbers);  // Output: [1, 2, 3]
console.log(person);   // Output: ["Alice", 30]

Arrays allow dynamic collections, while tuples enforce fixed-length structures with specific types.

Objects and Interfaces

Objects store key-value pairs. Interfaces define the structure of objects for type safety.

objects_interfaces.ts

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

let user: User = { name: "Bob", age: 25 };
console.log(user);  // Output: { name: "Bob", age: 25 }

Interfaces ensure objects adhere to predefined structures, improving code readability and maintainability.

Enums

Enums define a set of named constants. They improve code readability by replacing magic numbers or strings.

enums.ts

enum Status {
    Active = "ACTIVE",
    Inactive = "INACTIVE",
}

let currentStatus: Status = Status.Active;
console.log(currentStatus);  // Output: ACTIVE

Enums provide a structured way to manage constants, reducing errors and improving code clarity.

Type Aliases

Type aliases create custom types for reuse. They simplify complex type definitions and improve code organization.

type_aliases.ts

type Point = {
    x: number;
    y: number;
};

let origin: Point = { x: 0, y: 0 };
console.log(origin);  // Output: { x: 0, y: 0 }

Type aliases make code more readable by abstracting complex type definitions into reusable components.

Best Practices

Use const for Constants: Prefer const for immutable values
Explicit Types: Use explicit types for clarity in complex scenarios
Avoid var: Prefer let and const over var
Type Inference: Leverage type inference for simple cases
Interfaces for Objects: Use interfaces to define object structures

Source

TypeScript Variables Documentation

This tutorial covered TypeScript variables with practical examples. Implement these patterns to write safer, more maintainable code.

Author

My name is Jan Bodnar, and I am a passionate programmer with extensive programming experience. I have been writing programming articles since 2007. To date, I have authored over 1,400 articles and 8 e-books. I possess more than ten years of experience in teaching programming.

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