Quick Answer

Attempting to access undeclared properties

Understanding the Issue

TypeScript performs static type checking and will prevent access to properties that aren't declared in the type definition. This ensures type safety and catches typos.

The Problem

This code demonstrates the issue:

Typescript Error 
// Problem: Undeclared property
interface Person {
    name: string;
}
const p: Person = { name: "Alice" };
console.log(p.age); // Error

The Solution

Here's the corrected code:

Typescript Fixed 
// Solution 1: Add property to interface
interface Person {
    name: string;
    age?: number; // Optional
}

// Solution 2: Type assertion (when certain)
console.log((p as any).age); // Not recommended

// Solution 3: Use type with index signature
interface FlexiblePerson {
    name: string;
    [key: string]: any;
}

Key Takeaways

Ensure all accessed properties are properly declared in the type definition.