Definite Assignment Checks in TypeScript 5

• Before TypeScript 5:

let value: string;
console.log(value); // TS4.x: This was allowed but could lead to runtime undefined

• After TypeScript 5:

let value: string;
console.log(value); // TS5.x: Error - Variable 'value' is used before being assigned

// To fix, either initialize:
let value = "initial";

// Or use definite assignment assertion:
let value!: string;

• The ! is a promise to TypeScript that we'll assign a value before using it
• Use of ! should be avoided as it bypasses TypeScript's safety checks