JavaScript Notes

50 Tricky JavaScript Output Questions 2026 (With Answers)

50 tricky JavaScript output questions with detailed answers. Covers type coercion, hoisting, closures, event loop, promises, prototype, scope — ace your 2026 JS interview.

These questions are exactly what interviewers ask to test whether you truly understand JavaScript or just memorized the syntax. Read each one, predict the output, then reveal the answer.

Section 1: Type Coercion & Operators

Q1: Classic Plus Trap

js example

console.log(1 + "2" + 3);
console.log(1 + 2 + "3");
console.log("3" - 1);
console.log("3" * "2");

Output

"123"
"33"
2
6

Why? + left-to-right: 1+"2" → "12" → "12"+3 → "123". For - and *, strings are coerced to numbers.

Q2: typeof Surprises

js example

console.log(typeof null);
console.log(typeof undefined);
console.log(typeof NaN);
console.log(typeof function(){});
console.log(typeof []);
console.log(typeof {});

Output

"object"
"undefined"
"number"
"function"
"object"
"object"

Note: typeof null === "object" is a famous JavaScript bug kept for backward compatibility. Arrays and plain objects both return "object".

Q3: Loose Equality (==) Coercion

js example

console.log(0 == false);
console.log("" == false);
console.log(null == undefined);
console.log(null == 0);
console.log(NaN == NaN);
console.log([] == false);

Output

true
true
true
false
false
true

Key Rules:

null == undefined is true (special rule, no coercion)
null == 0 is false (null only equals undefined)
NaN equals nothing, not even itself
[] converts to "" then 0, which equals false

Q4: Short-Circuit Evaluation

Example

console.log(0 || "hello"); console.log(1 && "world"); console.log(false || null || 0 || "JS"); console.log(true && 42 && "final"); console.log(null ?? "default"); console.log(0 ?? "default");

Output

"hello"
"world"
"JS"
"final"
"default"
0

?? (Nullish Coalescing): Only falls back for null / undefined — NOT for 0 or "".

Q5: Unary Operators

js example

console.log(+"5");
console.log(+true);
console.log(+false);
console.log(+null);
console.log(+undefined);
console.log(+[]);
console.log(+{});

Output

5
1
0
0
NaN
0
NaN

Section 2: Hoisting

Q6: var Hoisting

js example

console.log(a);
var a = 10;
console.log(a);

Output

undefined
10

Q7: Function Hoisting Priority

js example

console.log(typeof foo);

var foo = "variable";
function foo() { return "function"; }

console.log(typeof foo);

Output

"function"
"string"

Why? Function declarations are hoisted before var declarations. After var foo = "variable" executes, it becomes a string.

Q8: Function Expression Not Hoisted

js example

console.log(bar());
var bar = function() { return "bar"; };

Output

TypeError: bar is not a function

Why? var bar is hoisted as undefined. Calling undefined() throws a TypeError.

Q9: let TDZ (Temporal Dead Zone)

js example

console.log(x);
let x = 5;

Output

ReferenceError: Cannot access 'x' before initialization

Q10: Nested Hoisting

js example

var x = 1;
function outer() {
  console.log(x);
  var x = 2;
  console.log(x);
}
outer();
console.log(x);

Output

undefined
2
1

Why? Local var x is hoisted inside outer(), shadowing global x with undefined until assigned.

Section 3: Closures & Scope

Q11: Closure Counter

Example

function outer() { let n = 0; return () => ++n; } const f1 = outer(); const f2 = outer(); console.log(f1(), f1(), f2(), f1());

Output

1 2 1 3

Q12: Loop with var

Example

const funcs = []; for (var i = 0; i < 3; i++) { funcs.push(() => i); } console.log(funcs[0](), funcs[1](), funcs[2]());

Output

3 3 3

Q13: Loop with let

Example

const funcs = []; for (let i = 0; i < 3; i++) { funcs.push(() => i); } console.log(funcs[0](), funcs[1](), funcs[2]());

Output

0 1 2

Q14: Variable Shadowing

js example

let val = "global";
function test() {
  console.log(val); // TDZ!
  let val = "local";
  console.log(val);
}
test();

Output

ReferenceError: Cannot access 'val' before initialization

Why? let val = "local" inside test() creates a TDZ from the start of the block — even though a global val exists, the local one shadows it immediately.

Q15: Lexical Scope

js example

let name = "Global";
function foo() { console.log(name); }
function bar() {
  let name = "Bar";
  foo();
}
bar();

Output

Global

Section 4: this Keyword

Q16: Method vs Function

Example

const obj = { name: "JS", getName: function() { return this.name; }, getNameArrow: () => this.name }; console.log(obj.getName()); console.log(obj.getNameArrow());

Output

"JS"
undefined

Why? Arrow functions don't have their own this. In the module/strict context, this.name outside any method is undefined.

Q17: Detached Method

js example

const person = {
  name: "Rahul",
  greet() { console.log("Hi, " + this.name); }
};

const greet = person.greet;
greet(); // 'this' is window/undefined
person.greet(); // 'this' is person

Output

Hi, undefined
Hi, Rahul

Q18: call / apply / bind

Example

Output

Hello, India!
Namaste, India.
Hey, India?

Section 5: Promises & Async

Q19: Promise Execution Order

Example

console.log("1"); setTimeout(() => console.log("2"), 0); Promise.resolve().then(() => console.log("3")); console.log("4");

Output

Rule: Sync → Microtasks (Promises) → Macrotasks (setTimeout).

Q20: Promise Chain

Example

Promise.resolve(1) .then(x => x + 1) .then(x => { throw new Error("boom"); }) .catch(e => console.log("Caught:", e.message)) .then(() => console.log("Recovered"));

Output

Caught: boom
Recovered

Q21: async/await Order

js example

async function fetchData() {
  console.log("A");
  const result = await Promise.resolve("B");
  console.log(result);
  console.log("C");
}

console.log("Start");
fetchData();
console.log("End");

Output

Start
A
End
B
C

Why? await pauses fetchData (schedules continuation as microtask) and returns control to the caller. "End" prints first, then the awaited microtask resumes.

Q22: Promise.all

Example

Promise.all([ Promise.resolve("A"), Promise.resolve("B"), Promise.reject("Error!") ]) .then(vals => console.log(vals)) .catch(err => console.log("Failed:", err));

Output

Failed: Error!

Rule: Promise.all rejects immediately on the first rejection.

Section 6: Arrays & Objects

Q23: Array Equality

js example

console.log([] == []);
console.log([] === []);
console.log({} === {});
const a = [];
const b = a;
console.log(a === b);

Output

false
false
false
true

Why? Objects/arrays are reference types. Two separate [] or {} are stored at different memory addresses. Only variables pointing to the same object are equal.

Q24: Object Key Coercion

Example

const obj = {}; obj[1] = "one"; obj["1"] = "ONE"; console.log(obj[1]); console.log(Object.keys(obj).length);

Output

"ONE"
1

Why? Object keys are always strings. 1 and "1" are the same key — the second assignment overwrites the first.

Q25: Array Spread + Push

Example

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

Output

[1, 2, 3]
[1, 2, 3, 4]

Spread creates a shallow copy — modifying copy doesn't affect arr.

Q26: delete Operator

js example

const obj = { a: 1, b: 2, c: 3 };
delete obj.b;
console.log(obj);
console.log("b" in obj);
console.log(obj.b);

Output

{ a: 1, c: 3 }
false
undefined

Section 7: Prototype & Inheritance

Q27: Prototype Chain Lookup

js example

function Animal(name) { this.name = name; }
Animal.prototype.speak = function() {
  return this.name + " speaks";
};

const dog = new Animal("Dog");
console.log(dog.speak());
console.log(dog.hasOwnProperty("name"));
console.log(dog.hasOwnProperty("speak"));

Output

Dog speaks
true
false

Q28: instanceof

js example

function Foo() {}
const f = new Foo();
console.log(f instanceof Foo);
console.log(f instanceof Object);
console.log([] instanceof Array);
console.log([] instanceof Object);

Output

true
true
true
true

Section 8: Miscellaneous Tricky

Q29: Comma Operator

js example

let x = (1, 2, 3);
console.log(x);

Output

Comma operator evaluates each expression and returns the last one.

Q30: String Immutability

Example

let str = "hello"; str[0] = "H"; console.log(str);

Output

"hello"

Why? Strings in JavaScript are immutable — character assignment silently fails in non-strict mode.

Q31: Chained Ternary

js example

const n = 15;
const result = n > 10 ? n > 20 ? "big" : "medium" : "small";
console.log(result);

Output

"medium"

Q32: Object Destructuring Default

js example

const { a = 10, b = 20 } = { a: 3, b: undefined };
console.log(a, b);

Output

3 20

Note: Destructuring defaults apply only when the value is undefined, NOT null.

Q33: Spread vs Rest

Example

function sum(...nums) { return nums.reduce((a, b) => a + b, 0); } console.log(sum(1, 2, 3, 4, 5)); console.log(sum(...[10, 20]));

Output

15
30

Q34: String Concatenation Trap

js example

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

Output

""
"[object Object]"
0

Why? [] converts to "". {} before + at statement start is parsed as empty block, then +[] becomes 0.

Q35: Number Conversions

js example

console.log(Number(""));
console.log(Number(" "));
console.log(Number("  123  "));
console.log(Number("123abc"));
console.log(Number(null));
console.log(Number(undefined));
console.log(Number(true));
console.log(Number(false));

Output

0
0
123
NaN
0
NaN
1
0

Common Traps Summary 🪤

Code	Output	Reason
`typeof null`	`"object"`	Historic JS bug
`NaN === NaN`	`false`	NaN is not equal to itself
`null == undefined`	`true`	Special JS rule
`null === undefined`	`false`	Strict: different types
`[] == false`	`true`	[] → "" → 0 → false
`0.1 + 0.2 === 0.3`	`false`	IEEE 754 float precision
`"5" - 3`	`2`	String coerced to number
`"5" + 3`	`"53"`	Number coerced to string

Quick Self-Test

Predict these without running:

js example

console.log(0.1 + 0.2 === 0.3);        // ?
console.log(typeof typeof 42);          // ?
console.log(!!"");                      // ?
console.log(!!0);                       // ?
console.log(!!"hello");                 // ?
console.log(+"");                       // ?
console.log(undefined + 1);            // ?
console.log(null + 1);                  // ?

Output

false
"string"
false
false
true
0
NaN
1

Interview Tips for Tricky Questions 🎯

Pause and think step-by-step — don't blurt out the first answer.
Explain type coercion rules aloud as you go.
State the phase — "during hoisting, var x gets undefined..."
Mention edge cases — shows depth (e.g., null + 1 = 1, not NaN).
Practice 5 output questions daily — muscle memory for interviews.

Exam Focus

Revise definitions, diagrams, examples, and short-answer points for 50 Tricky JavaScript Output Questions 2026 (With Answers).

Interview Use

Prepare one clear explanation, one practical example, and one common mistake for this JavaScript Master Course topic.

Search Terms

javascript-complete-guide, javascript master course, javascript, complete, guide, interview, tricky, output

Continue learning this concept

InterviewTop 30 JavaScript Closure Interview Questions & Answers 2026Master closure interview questions with detailed answers, code examples, outputs, common traps, and tips. Covers IIFE, loop problem, data privacy, currying, memoization.InterviewTop 30 JavaScript Coding Interview Questions 2026 (With Solutions)Top 30 JavaScript coding interview questions with step-by-step solutions. Covers arrays, strings, objects, recursion, algorithms, and problem-solving patterns for 2026 interviews.InterviewTop 25 JavaScript Event Loop Interview Questions & Answers 2026Master JavaScript event loop interview questions with call stack, microtasks, macrotasks, Promise order, async/await execution, and real output prediction examples.InterviewTop 30 JavaScript Promise Interview Questions & Answers 2026Master JavaScript Promise interview questions with detailed answers. Covers Promise states, chaining, Promise.all/race/allSettled/any, async/await, error handling, and output questions.InterviewTop 50 JavaScript Interview Questions & Answers 2026Complete JavaScript interview guide 2026. Covers 50 most asked questions on closures, hoisting, promises, prototypes, async/await, event loop, DOM, and ES6+ features.Interview PreparationJavaScript Interview — Behavioral Questions and Soft Skills Guide 2026Master behavioral interview questions for JavaScript developer roles. Learn how to structure answers about teamwork, debugging challenges, code reviews, and technical decision-making using the STAR method.

PreviousTop 30 JavaScript Promise Interview Questions & Answers 2026 NextBuild a JavaScript Calculator - Step by Step Project Tutorial