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Type System Deep Dive: Inference and Compatibility

Advanced
130 XP
50 min
Lesson Content

TypeScript Type System Deep Dive

Understanding how TypeScript's type system works under the hood helps you write better, more type-safe code.

Type Inference

// TypeScript infers types
let x = 10; // inferred as number
let y = "hello"; // inferred as string

// Explicit vs inferred
let a: number = 10; // explicit
let b = 10; // inferred

Structural Typing

// TypeScript uses structural typing (duck typing)
interface Point { x: number; y: number; }

let point: Point = { x: 10, y: 20 };
let point2 = { x: 10, y: 20, z: 30 }; // Also compatible!

Type Compatibility

// Covariance, contravariance, and invariance
// Functions are contravariant in parameters, covariant in return

Type Widening and Narrowing

let x = "hello"; // type: string (widened)
const y = "hello"; // type: "hello" (literal, not widened)
Example Code

Understanding type inference and compatibility

// Type inference examples
let inferredNumber = 42; // TypeScript infers: number
let inferredString = "hello"; // TypeScript infers: string

// Explicit types
let explicit: number = 42;
let explicitString: string = "hello";

console.log(`Inferred: ${typeof inferredNumber}, Explicit: ${typeof explicit}`);

Expected Output:

Inferred: number, Explicit: number
Study Tips
  • •Read the theory content thoroughly before practicing
  • •Review the example code to understand key concepts
  • •Proceed to the Practice tab when you're ready to code