এরর হ্যান্ডলিং (Error Handling)

// Basic try/catch
try {
  let data = JSON.parse("not valid json");
  console.log(data);
} catch (error) {
  console.log("Caught an error!");
  console.log(error.name);    // "SyntaxError"
  console.log(error.message); // "Unexpected token 'o'..."
}

// finally — always runs, error or not
function readFile(name) {
  console.log(`Opening ${name}...`);
  try {
    if (name === "missing.txt") {
      throw new Error("File not found!");
    }
    console.log(`Reading ${name}... done!`);
    return "file contents";
  } catch (error) {
    console.log(`Error: ${error.message}`);
    return null;
  } finally {
    console.log(`Closing ${name}. (always runs)`);
  }
}

readFile("data.txt");
console.log("---");
readFile("missing.txt");

Caught an error!
SyntaxError
Unexpected token 'o', "not valid json" is not valid JSON
Opening data.txt...
Reading data.txt... done!
Closing data.txt. (always runs)
---
Opening missing.txt...
Error: File not found!
Closing missing.txt. (always runs)

// Throwing errors for validation
function divide(a, b) {
  if (typeof a !== "number" || typeof b !== "number") {
    throw new TypeError("Both arguments must be numbers");
  }
  if (b === 0) {
    throw new RangeError("Cannot divide by zero");
  }
  return a / b;
}

try {
  console.log(divide(10, 2));      // 5
  console.log(divide(10, 0));      // throws!
} catch (e) {
  console.log(`${e.name}: ${e.message}`);
}

// Custom error class
class ValidationError extends Error {
  constructor(field, message) {
    super(message);
    this.name = "ValidationError";
    this.field = field;
  }
}

function createUser(name, age) {
  if (!name) throw new ValidationError("name", "Name is required");
  if (age < 0) throw new ValidationError("age", "Age cannot be negative");
  return { name, age };
}

try {
  createUser("", 25);
} catch (e) {
  if (e instanceof ValidationError) {
    console.log(`Validation failed on '${e.field}': ${e.message}`);
  } else {
    throw e; // re-throw unexpected errors
  }
}

5
RangeError: Cannot divide by zero
Validation failed on 'name': Name is required

// A reusable safe parser
function safeJsonParse(text, fallback = null) {
  try {
    return JSON.parse(text);
  } catch {
    return fallback;
  }
}

// Works perfectly with valid JSON
let data = safeJsonParse('{"name": "Laboni", "level": 5}');
console.log(data); // { name: "Laboni", level: 5 }

// Returns fallback for invalid JSON instead of crashing
let bad = safeJsonParse("not json", { name: "Unknown" });
console.log(bad); // { name: "Unknown" }

// Guard clause pattern — validate early, fail fast
function processOrder(order) {
  if (!order) throw new Error("Order is required");
  if (!order.items?.length) throw new Error("Order must have items");
  if (order.total < 0) throw new Error("Total cannot be negative");

  // If we get here, we know the data is good!
  console.log(`Processing order: ${order.items.length} items, $${order.total}`);
}

try {
  processOrder({ items: ["book", "pen"], total: 24.99 });
  processOrder({ items: [], total: 0 });
} catch (e) {
  console.log(`Failed: ${e.message}`);
}

{ name: "Laboni", level: 5 }
{ name: "Unknown"
Processing order: 2 items, $24.99
Failed: Order must have items