The most common JavaScript interview format: read a snippet, say what it prints and why. Try each one before looking at the output.

1. Hoisting

JavaScript

var x = 1;
function test() {
  console.log(x);
  var x = 2;
  console.log(x);
}
test();
console.log(typeof hoisted, typeof expr);
function hoisted() {}
var expr = function () {};

Output

undefined
2
function undefined

The local var x is hoisted to the top of test (as undefined) and shadows the global. expr is a var still holding undefined when typeof runs.

2. Closures in loops

JavaScript

const fns = [];
for (var i = 0; i < 3; i++) fns.push(() => i);
const fns2 = [];
for (let i = 0; i < 3; i++) fns2.push(() => i);
console.log(fns.map((f) => f()), fns2.map((f) => f()));

Output

[ 3, 3, 3 ] [ 0, 1, 2 ]

3. this

JavaScript

const obj = {
  name: "obj",
  regular() { return this.name; },
  arrow: () => typeof this,
  nested() {
    const inner = () => this.name;     // arrow borrows nested()'s this
    function plain() { return this; }  // plain call → undefined (strict)
    return [inner(), plain()];
  },
};
const { regular } = obj;
console.log(obj.regular(), obj.arrow(), obj.nested());
try { regular(); } catch (e) { console.log(e.constructor.name); }

Output

obj undefined [ 'obj', undefined ]
TypeError

4. Event loop ordering

JavaScript

console.log(1);
setTimeout(() => console.log(2), 0);
Promise.resolve().then(() => {
  console.log(3);
  setTimeout(() => console.log(4), 0);
  Promise.resolve().then(() => console.log(5));
});
(async () => {
  console.log(6);
  await undefined;
  console.log(7);
})();
console.log(8);

Output

Sync first (1, 6, 8). Then microtasks in queue order: the then (3, which queues 5 and timer 4), the await continuation (7), then 5. Finally macrotasks in order: 2, then 4.

5. Coercion

JavaScript

console.log(1 < 2 < 3, 3 > 2 > 1);
console.log("5" + 3, "5" - 3, "5" * "2", true + true);
console.log([] + null + 1, [2] * [3], {} + []);
console.log(0 || null || "" || undefined, 1 && "a" && 0);

Output

true false
53 2 10 2
null1 6 [object Object]
undefined 0

3 > 2 > 1 → true > 1 → 1 > 1 → false. [] + null + 1 → "" + "null" → "null" + 1. || returns the last operand when all are falsy; && returns the first falsy one.

6. References and copies

JavaScript

let a = { n: 1 };
let b = a;
a.n = 2;
a = { n: 3 };
console.log(b.n);

const arr = [1, 2, 3];
const copy = arr;
copy.push(4);
const spread = [...arr];
spread.push(5);
console.log(arr.length, spread.length);

function change(o) { o.value = "changed"; o = { value: "replaced" }; }
const item = { value: "original" };
change(item);
console.log(item.value);

Output

2
4 5
changed

7. Promises

JavaScript

const p = new Promise((resolve, reject) => {
  console.log("A");
  resolve("B");
  reject("C");
  console.log("D");
});
p.then(console.log).catch(() => console.log("never"));
Promise.reject(new Error("E"))
  .then(() => console.log("skipped"))
  .catch((e) => e.message)
  .then((m) => console.log(m + "!"))
  .finally(() => console.log("F"));
console.log("G");

Output

A
D
G
B
E!
F

8. Mixed bag

JavaScript

console.log(typeof typeof 1, typeof [] === typeof null);
console.log([..."abc"].map(parseInt));
console.log(0.1 + 0.2 == 0.3, 0.5 + 0.25 == 0.75);
const x = { a: 1 }, y = { a: 1 };
console.log(x == y, JSON.stringify(x) === JSON.stringify(y));
console.log([1, 2, 3].indexOf(3) > -1 ? "found" : "missing", [10, 1, 3].sort().join());
function Foo() { return this; }
console.log(new Foo() instanceof Foo, Foo());

Output

string true
[ NaN, NaN, NaN ]
false true
false true
found 1,10,3
true undefined