JavaScript Output Questions with Answers

function show() {
  console.log(score);
  var score = 90;
  console.log(score);
}
show();

undefined
90

var score is hoisted to the top of the function and initialised with undefined. Only the assignment = 90 stays where it's written.

function show() {
  console.log(marks);
  let marks = 85;
}
show();

ReferenceError: Cannot access 'marks' before initialization

let is hoisted too, but it stays uninitialised in the temporal dead zone until its line runs. Reading it early throws instead of giving undefined.

for (var i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 0);
}

3
3
3

All three callbacks close over the same var i. They run after the loop ends, when i is 3.

for (let i = 0; i < 3; i++) {
  setTimeout(() => console.log(i), 0);
}

0
1
2

let creates a new binding for each iteration, so each callback has its own i.

const course = {
  title: "React",
  describe() {
    return `Course: ${this.title}`;
  },
};
console.log(course.describe());

Course: React

Called as course.describe(), so this is the object before the dot: course.

"use strict";
const course = {
  title: "React",
  describe() {
    return this.title;
  },
};

const describe = course.describe;
try {
  console.log(describe());
} catch (error) {
  console.log(error.name + ": " + error.message);
}

TypeError: Cannot read properties of undefined (reading 'title')

describe() is a plain call with nothing before the dot. In strict mode (and in all modules and classes), this is then undefined.

console.log([] + {});
console.log(String([]), String({}));

[object Object]
 [object Object]

+ with objects converts both to primitives (strings here): [] becomes "" and {} becomes "[object Object]". The empty string plus that is just "[object Object]". The second line starts with an empty string, which is why there's a leading space.

console.log([] == ![]);
console.log([] == []);
console.log([] == "");

true
false
true

![] is false (arrays are truthy). Then [] == false converts both sides to numbers: [] becomes "" then 0, and false becomes 0, so 0 == 0. Two separate arrays are different objects, so [] == [] is false.

console.log("A");
setTimeout(() => console.log("B"), 0);
Promise.resolve().then(() => console.log("C"));
console.log("D");

A
D
C
B

Synchronous code runs first (A, D). Then the microtask queue (promise callbacks: C), and only then the next macrotask (the timer: B).

console.log("1");
new Promise((resolve) => {
  console.log("2");
  resolve();
  console.log("3");
}).then(() => console.log("5"));
console.log("4");

1
2
3
4
5

The function passed to new Promise runs immediately. resolve() doesn't stop it, so 3 still prints. Only the .then callback waits for the microtask queue.