06-error-handling-and-memory.md

Error Handling & Memory

Temporal Dead Zone

Accessing let/const bindings before initialization throws.

// console.log(x); // ReferenceError
let x = 1;

Exception Handling

Use try/catch/finally.

try {
  JSON.parse("not json");
} catch (err) {
  console.error("parse failed:", err.message);
} finally {
  // cleanup
}

Throw your own errors:

function mustBeNumber(x) {
  if (typeof x !== "number") throw new TypeError("x must be a number");
  return x;
}

Shadowing

Shadowing is declaring a variable with the same name in an inner scope.

const x = 1;
{
  const x = 2; // shadows outer x
  console.log(x); // 2
}

Illegal Shadowing

In JS, let/const cannot be shadowed by var in the same or nested scope in ways that break rules.

// This is illegal:
// let y = 1;
// {
//   var y = 2; // SyntaxError
// }

Coercion

Coercion is implicit type conversion.

console.log("5" + 1); // "51" (string concatenation)
console.log("5" - 1); // 4 (numeric)
console.log(Boolean("")); // false

Prefer explicit conversions when clarity matters:

const n = Number("42");

Memory Leak

Common causes:

• keeping references in long-lived structures
• forgotten timers / intervals
• DOM references in browsers

// Browser example: keep references minimal and remove listeners.