How to fix Rest parameter must be of an array type in TypeScript

// ❌ WRONG - primitive type
function sum(...numbers: number) {
  // Error: Rest parameter must be of an array type
}

// ✅ CORRECT - array type
function sum(...numbers: number[]) {
  return numbers.reduce((a, b) => a + b, 0);
}

// Before (error):
function logMessages(...messages: string) {
  messages.forEach(msg => console.log(msg));
}

// After (fixed):
function logMessages(...messages: string[]) {
  messages.forEach(msg => console.log(msg));
}

// Or using Array<T> syntax:
function logMessages(...messages: Array<string>) {
  messages.forEach(msg => console.log(msg));
}

// Tuple type for exactly two arguments
function processPair(...pair: [string, number]) {
  const [name, value] = pair;
  console.log(`${name}: ${value}`);
}

// Variable-length tuple with rest element
function processWithOptional(...args: [string, ...number[]]) {
  const [name, ...values] = args;
  console.log(`${name} has ${values.length} values`);
}

// ❌ WRONG - rest parameter not last
function process(a: number, ...rest: number[], b: string) {
  // Error: A rest parameter must be last in a parameter list
}

// ✅ CORRECT - rest parameter is last
function process(a: number, b: string, ...rest: number[]) {
  // Works correctly
}

// Fixed function
function calculateAverage(...numbers: number[]) {
  if (numbers.length === 0) return 0;
  const sum = numbers.reduce((a, b) => a + b, 0);
  return sum / numbers.length;
}

// Test cases
console.log(calculateAverage());           // 0
console.log(calculateAverage(10));         // 10
console.log(calculateAverage(10, 20));     // 15
console.log(calculateAverage(10, 20, 30)); // 20

// Works with spread syntax too
const scores = [85, 90, 78, 92];
console.log(calculateAverage(...scores));  // 86.25