Module 24 // Core JavaScript

Error Handling & Custom Errors

Module Objective

Built-in error types, custom errors, propagation, stack traces

Mental Model Realtime Simulation

INTERACTIVE_CANVAS

Editor_Pane

Console_Output

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Practical Code Examples

// Example 1

// 1. Specific Error Types
try {
  // decodeURIComponent("%"); // ❌ URIError
  // const x = y;            // ❌ ReferenceError
  // JSON.parse("{bad}");    // ❌ SyntaxError
} catch (err) {
  if (err instanceof ReferenceError) {
    console.log("Missing variable!");
  } else {
    console.log("General error:", err.message);
  }
}

💡 JavaScript has specific built-in error types. Using `instanceof` allows you to handle different failures in different ways.

// Example 2

// 2. Re-throwing Errors
function process() {
  try {
    throw new Error("Database Fail");
  } catch (err) {
    console.log("Logging error internally...");
    throw err; // Send it up to the caller
  }
}

try {
  process();
} catch (e) {
  console.log("Caller caught:", e.message);
}

💡 Sometimes you want to log an error but still let the calling function know that something went wrong.

// Example 3

// 3. The Power of Finally
function test() {
  try {
    return "Result";
  } finally {
    console.log("Cleanup complete");
  }
}

console.log(test());

💡 Even if you `return` from inside the `try` block, the `finally` block **is guaranteed to run**. This is essential for closing files or database connections.

Engine & Memory Architecture

Errors & The Stack

1. Stack Traces:

When an Error object is created, it captures the current state of the Call Stack.
This 'trace' is stored as a string in the stack property in the Heap.

2. Exception Propagation:

If an error isn't caught, it "bubbles up" the Call Stack.
Each frame is popped until a catch block is found.
If none is found, the engine terminates and prints the stack trace to the console.

3. Performance:

Creating and throwing errors is relatively expensive (due to stack capture).
Rule: Use Errors for exceptional cases, not for regular control flow (like if/else).

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