Quick Answer

Type mismatch in assignments or returns

Understanding the Issue

TypeScript uses structural typing but still enforces type compatibility. This error occurs when trying to assign or return a value of the wrong type.

The Problem

This code demonstrates the issue:

Typescript Error 
// Problem: Type mismatch
interface Animal { name: string; }
interface Dog extends Animal { breed: string; }

let animal: Animal = { name: "Rex" };
let dog: Dog = animal; // Error

The Solution

Here's the corrected code:

Typescript Fixed 
// Solution 1: Correct type assignment
let dog: Dog = { name: "Rex", breed: "Labrador" };

// Solution 2: Type assertion (when certain)
let dog = animal as Dog; // Unsafe

// Solution 3: Use intersection type
let animalDog: Animal & { breed?: string } = animal;

Key Takeaways

Ensure types are compatible in assignments or use proper type assertions.