TypeScript Simple Types

TypeScript Simple Types: Primitive Data Types & Explicit Annotations

TypeScript supports all primitive JavaScript types—boolean, number, string, bigint, symbol, undefined, and null—and allows you to explicitly annotate variables or rely on automatic type inference.

Core Primitive Types Overview

1. string: Textual data (e.g., 'hello', "world", `template string`).
2. number: Floating point numbers and integers (e.g., 42, 3.14159, 0xFF).
3. boolean: Truth values (true or false).
4. bigint: Large integers beyond Number.MAX_SAFE_INTEGER (e.g., 9007199254740991n).
5. symbol: Unique immutable primitive values created via Symbol().

Practical Code Example

// Explicit type annotations
const appName: string = "Analytics Engine";
const maxConnections: number = 100;
const isDebugEnabled: boolean = false;
const globalTransactionId: bigint = 9007199254740993n;
const processKey: symbol = Symbol("processKey");

function calculateTotalTax(amount: number, taxRate: number): number {
  return amount * taxRate;
}

const itemPrice: number = 299.99;
const rate: number = 0.15;
const totalTax: number = calculateTotalTax(itemPrice, rate);

console.log(`[${appName}] Total Tax on \$${itemPrice}: \$${totalTax.toFixed(2)}`);

Best Practices & Gotchas

• Use Lowercase Primitives: Always annotate primitive types with lowercase names (string, number, boolean), not capitalized Object wrappers (String, Number, Boolean).
• Don't Redundantly Annotate Simple Inferences: For simple assignments like const count = 5;, explicit const count: number = 5; is unnecessary.
• Beware of NaN: NaN is technically of type number in TypeScript/JavaScript; always check Number.isNaN() when parsing numbers.

Self-Check Challenge

Declare a variable userAge with explicit type number and set it to 25. Attempt to assign "twenty-five" to it and note the compiler error message.