How to fix Returned expression type is not assignable to return type in TypeScript

// Error: Type 'string' is not assignable to type 'number'
function getNumber(): number {
  return "5"; // WRONG - returning string, not number
}

// CORRECT - return a number
function getNumber(): number {
  return 5; // number = OK
}

// OR convert string to number
function parseNumber(): number {
  const value = "5";
  return parseInt(value, 10); // convert string to number
}

interface User {
  id: number;
  name: string;
  email: string;
}

function createUser(): User {
  // ERROR - missing required properties
  return { name: "John" };

  // CORRECT - includes all required properties
  // return { id: 1, name: "John", email: "[email protected]" };
}

function getName(): string {
  const name: string | null = getUserName(); // might be null

  // ERROR - might return null
  // return name;

  // CORRECT - provide default value
  return name ?? "Guest";
}

// OR update return type to include null
function getNameOrNull(): string | null {
  const name: string | null = getUserName();
  return name; // OK - return type includes null
}

function getValue(): number {
  const value: unknown = 42;

  // TypeScript doesn't know this is a number
  // return value; // ERROR

  // If you're certain it's a number, use 'as'
  return value as number; // OK
}

function identity<T>(value: T): T {
  return value;
}

// TypeScript might infer the wrong type
const result1 = identity({ name: "John" }); // inferred as { name: string }

// Explicitly specify the type to be sure
interface User {
  name: string;
  age: number;
}

const result2 = identity<User>({ name: "John", age: 30 }); // must match User fully

{
  "compilerOptions": {
    "strict": true,        // Enables strictNullChecks
    "strictNullChecks": true   // null/undefined not allowed unless explicitly in type
  }
}

function getOptionalName(): string {
  const user = { name: undefined };
  return user.name; // ERROR: name might be undefined
  // return user.name ?? ""; // CORRECT
}

// WRONG - missing Promise wrapper
async function fetchData(): string {
  const response = await fetch("/api/data");
  return response.json(); // ERROR: returns Promise<string>, not string
}

// CORRECT - return type is Promise<string>
async function fetchData(): Promise<string> {
  const response = await fetch("/api/data");
  return response.json(); // OK: returns Promise<string>
}