JavaScript Internals: The Event Loop, Closures, and the Temporal Dead Zone

Three JavaScript concepts that trip up even experienced developers.
Understanding them makes debugging much faster.

The Temporal Dead Zone (TDZ)

console.log(x)  // undefined (hoisted + initialized)
var x = 5

console.log(y)  // ReferenceError: Cannot access 'y' before initialization
let y = 5       // TDZ ends here

var hoists and initializes to undefined. let/const hoist but stay uninitialized until declaration.

Closure Fundamentals

function makeCounter() {
  let count = 0
  return {
    increment: () => ++count,
    value: () => count,
  }
}

const counter = makeCounter()
counter.increment()  // 1
counter.increment()  // 2
// count is private — only accessible through the returned methods

The classic loop bug:

for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 0)
}
// Prints: 3, 3, 3  — all share same var i

for (let i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 0)
}
// Prints: 0, 1, 2  — let creates new binding per iteration

The Event Loop

console.log('sync 1')
setTimeout(() => console.log('macrotask'), 0)
Promise.resolve().then(() => console.log('microtask'))
console.log('sync 2')

// Output: sync 1, sync 2, microtask, macrotask

Priority: synchronous > microtasks (Promises) > macrotasks (setTimeout)

Microtasks drain completely before any macrotask runs:

Promise.resolve()
  .then(() => console.log('micro 1'))
  .then(() => console.log('micro 2'))
  .then(() => console.log('micro 3'))

setTimeout(() => console.log('macro'), 0)

// Output: micro 1, micro 2, micro 3, macro

Async/Await Under the Hood

// These compile to the same thing:
async function getData() {
  const result = await fetch('/api/data')
  return result.json()
}

function getData() {
  return fetch('/api/data').then(r => r.json())
}

await pauses the function, lets other code run, then resumes when the Promise resolves.

Avoid Blocking the Event Loop

// Bad: blocks all other JS for seconds
function heavyWork() {
  const end = Date.now() + 3000
  while (Date.now() < end) {}  // nothing else can run
}

// Better: yield to event loop periodically
async function heavyWork(items) {
  for (let i = 0; i < items.length; i++) {
    process(items[i])
    if (i % 100 === 0) {
      await new Promise(r => setTimeout(r, 0))  // yield
    }
  }
}

Prototype Chain

class Animal {
  speak() { return `${this.name} speaks` }  // on Animal.prototype
}

class Dog extends Animal {
  constructor(name) {
    super()
    this.name = name  // own property
  }
}

const rex = new Dog('Rex')
rex.speak()  // found on Animal.prototype via chain
rex.hasOwnProperty('name')   // true
rex.hasOwnProperty('speak')  // false

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