JavaScript Interview: Output Prediction Questions

JavaScript Interview: Output Prediction Questions

Output prediction questions are a favorite interview format. Here are examples.

Q1: Event Loop

console.log("1");
setTimeout(() => console.log("2"), 0);
Promise.resolve().then(() => console.log("3"));
console.log("4");
``
**Answer**: 1, 4, 3, 2 (sync > microtask > macrotask).

### Q2: Hoisting

```js
console.log(a);
var a = 5;
``
**Answer**: undefined (var is hoisted with undefined).

### Q3: TDZ

```js
console.log(b);
let b = 5;
``
**Answer**: ReferenceError (let is in TDZ).

### Q4: Closures Loop

```js
for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 0);
}
``
**Answer**: 3, 3, 3 (var shares one i).

### Q5: this Keyword

```js
const obj = { x: 10, foo: () => console.log(this.x) };
obj.foo();
``
**Answer**: undefined (arrow inherits lexical this).

### Q6: Promise Chaining

```js
Promise.resolve(1).then(n => n + 1).then(n => console.log(n));
``
**Answer**: 2 (each .then passes its return).

### Q7: async/await

```js
async function foo() {
  console.log("1");
  await Promise.resolve();
  console.log("2");
}
foo();
console.log("3");
``
**Answer**: 1, 3, 2 (await pauses, rest is microtask).

### How to Prepare

1. Know the event loop order: sync > microtask > macrotask.
2. Know hoisting: var (undefined), let/const (TDZ).
3. Know closures: capture variable, not value.
4. Know `this`: 5 binding rules.
5. Practice 20+ output prediction questions.

### The Takeaway

Output prediction questions test: event loop (sync > microtask > macrotask), hoisting (var vs let/const), closures (loop bug), this keyword (arrow vs regular), promise chaining (return values), and async/await (microtask scheduling). Practice 20+ questions.