How to read a function without running it

The Signature Is the Contract

Before you read a function's body, fully understand its signature. The signature tells you what the function expects and what it promises to give back — that's 80% of what you need.

Dissecting a Signature

// What does this function do before reading the body?
async function fetchUserOrders(
  userId: string,
  options: { limit?: number; status?: 'pending' | 'shipped' | 'delivered' }
): Promise

// Reading the signature:
// - Takes a userId (string, required) — likely a DB lookup key
// - Takes options (object, optional fields) — filtering/pagination
// - Returns a Promise of Order array — async DB operation
// Conclusion: fetches orders for one user, possibly filtered

Pure vs Side-Effect Functions

A function that only uses its parameters and returns a value is pure — easy to reason about in isolation. A function that reads/writes files, makes network calls, or mutates external state has side effects — you need context to fully understand it.

// Pure — reads only its inputs
function calculateDiscount(price: number, percent: number): number {
  return price * (1 - percent / 100);
}

// Side effect — mutates external state
function applyDiscount(cart: Cart, percent: number): void {
  cart.items.forEach(item => {
    item.price = calculateDiscount(item.price, percent);
  });
}

Identify which category a function belongs to before diving into its body — it determines how much surrounding context you need to understand it correctly.