99 JavaScript Tricky Questions

Certainly! Here are 10 tricky JavaScript questions:

Variable Declaration and Initialization:
```
 let x = 5, y = 10;
 console.log(x + y);
```
What is the output, and why?
Truthy or Falsy:
```
 console.log([] == false);
```
What is the output, and why?

Scope and Variable Hoisting:

 var a = 10;
 (function () {
    console.log(a);
    var a = 20;
 })();

What is the output, and why?

Type Coercion:
```
 console.log("5" - 2);
```
What is the output, and why?

Closures:

 for (var i = 1; i <= 5; i++) {
    setTimeout(function() {
       console.log(i);
    }, 1000);
 }

What is the output, and why?

Prototype Chain:

 function Person() {}
 Person.prototype.walk = function() { console.log("walking"); };
 var john = new Person();
 john.walk();

What is the output, and why?

Async/Await:

 async function foo() {
    console.log(1);
    await Promise.resolve();
    console.log(2);
 }
 foo();
 console.log(3);

What is the output, and why?

this in Arrow Functions:

 const obj = {
    value: 100,
    getValue: () => console.log(this.value)
 };
 obj.getValue();

What is the output, and why?

Strict Mode:

 "use strict";
 function strictModeExample() {
    undeclaredVar = 10;
    console.log(undeclaredVar);
 }
 strictModeExample();

What is the output, and why?

Event Bubbling:

<div id="parent">
   <button id="child">Click me</button>
</div>
<script>
   document.getElementById('parent').addEventListener('click', function() {
      console.log('Parent clicked');
   });
   document.getElementById('child').addEventListener('click', function(e) {
      console.log('Child clicked');
      e.stopPropagation();
   });
</script>

What happens when you click the button, and why?

Certainly! Here are 20 more tricky JavaScript questions:

Global Object:

var a = 1;
console.log(window.a === a);

What is the output, and why?

NaN Comparison:
```
console.log(NaN === NaN);
```
What is the output, and why?
Array Equality:
```
console.log([1] == [1]);
```
What is the output, and why?

Hoisting in Functions:

function example() {
  console.log(bar);
  var bar = 10;
  console.log(bar);
}
example();

What is the output, and why?

Default Parameter Value:

function greet(name = "Guest") {
  console.log("Hello, " + name);
}
greet();
greet("John");

What is the output, and why?

typeof Operator:

console.log(typeof null);
console.log(typeof undefined);

What is the output, and why?

== vs ===:

console.log(1 == "1");
console.log(1 === "1");

What is the output for each line, and why?

instanceof Operator:

function Car() {}
const myCar = new Car();
console.log(myCar instanceof Car);

What is the output, and why?

Object Property Access:

var obj = { x: 10, y: 20 };
var { x, y } = obj;
console.log(x, y);

What is the output, and why?

setTimeout and this:

const obj = {
  count: 0,
  increment: function() {
    setTimeout(function() {
      console.log(this.count);
    }, 1000);
  }
};
obj.increment();

What is the output, and why?

Function Declarations vs Expressions:

console.log(foo());
console.log(bar());
function foo() { return "Hello, I'm foo!"; }
var bar = function() { return "Hello, I'm bar!"; };

What is the output, and why?

Promise Chaining:

Promise.resolve(1)
  .then((res) => {
    console.log(res);
    return 2;
  })
  .catch((err) => console.error(err))
  .then((res) => console.log(res));

What is the output, and why?

delete Operator:

const obj = { x: 10, y: 20 };
delete obj.x;
console.log(obj.x);

What is the output, and why?

Template Literals:

const name = "John";
const age = 30;
console.log(`My name is ${name} and I am ${age} years old.`);

What is the output, and why?

Object.keys and Property Order:

const obj = { a: 1, b: 2, c: 3 };
console.log(Object.keys(obj));

What is the output, and why?

Function Execution Context:

const a = 10;
function example() {
  console.log(a);
  const a = 20;
}
example();

What is the output, and why?

Array.prototype.map:

const arr = [1, 2, 3];
const result = arr.map(function(value) {
  return value * 2;
});
console.log(result);

What is the output, and why?

JSON.stringify:

const obj = { x: 10, y: undefined, z: function() {} };
console.log(JSON.stringify(obj));

What is the output, and why?

Regular Expressions:

const regex = /ab+c/;
const str = "abbc";
console.log(regex.test(str));

What is the output, and why?

parseInt Function:

console.log(parseInt("10.5"));

What is the output, and why?

Function.prototype.bind:

const obj1 = { x: 10 };
const obj2 = { x: 20 };
function getX() {
  return this.x;
}
const boundGetX1 = getX.bind(obj1);
const boundGetX2 = getX.bind(obj2);
console.log(boundGetX1(), boundGetX2());

What is the output, and why?

Array.isArray:

console.log(Array.isArray([]));
console.log(Array.isArray({}));

What is the output, and why?

Arrow Functions and this:

const obj = {
  count: 0,
  increment: function() {
    setTimeout(() => {
      console.log(this.count);
    }, 1000);
  }
};
obj.increment();

What is the output, and why?

Map Object Keys:

const myMap = new Map();
const key1 = { a: 1 };
const key2 = { b: 2 };
myMap.set(key1, 'value1');
myMap.set(key2, 'value2');
console.log(myMap.get(key1));

What is the output, and why?

Event Delegation:

<ul id="list">
  <li>Item 1</li>
  <li>Item 2</li>
  <li>Item 3</li>
</ul>
<script>
  document.getElementById('list').addEventListener('click', function(event) {
    console.log(event.target.innerText);
  });
</script>

What happens when you click on an item, and why?

Object.create:

const person = {
  greet: function() {
    console.log('Hello!');
  }
};
const john = Object.create(person);
john.name = 'John';
john.greet();

What is the output, and why?

Number Object:

console.log(typeof 42);
console.log(typeof new Number(42));

What is the output, and why?

IIFE (Immediately Invoked Function Expression):

(function() {
  var a = b = 5;
})();
console.log(b);
console.log(a);

What is the output, and why?

arguments Object:

function sum() {
  console.log(arguments.length);
}
sum(1, 2, 3);

What is the output, and why?

setTimeout and this in Non-Arrow Function:

const obj = {
  count: 0,
  increment: function() {
    setTimeout(function() {
      console.log(this.count);
    }.bind(this), 1000);
  }
};
obj.increment();

What is the output, and why?

instanceof and Inheritance:

function Animal() {}
function Dog() {}
Dog.prototype = Object.create(Animal.prototype);
const myDog = new Dog();
console.log(myDog instanceof Dog);
console.log(myDog instanceof Animal);

What is the output, and why?

Multiple catch Blocks:

try {
  throw new Error('Error occurred');
} catch (e) {
  console.log('Catch block 1:', e.message);
} catch (e) {
  console.log('Catch block 2:', e.message);
}

What is the output, and why?

WeakMap and Garbage Collection:

let obj = {};
const weakMap = new WeakMap();
weakMap.set(obj, 'value');
obj = null;
console.log(weakMap.get(obj));

What is the output, and why?

Array.prototype.reduce:

const numbers = [1, 2, 3];
const sum = numbers.reduce(function(acc, num) {
  return acc + num;
}, 0);
console.log(sum);

What is the output, and why?

Promise.all:

const promise1 = Promise.resolve(1);
const promise2 = new Promise((resolve) => setTimeout(() => resolve(2), 1000));
const promise3 = 3;
Promise.all([promise1, promise2, promise3])
  .then((values) => console.log(values))
  .catch((err) => console.error(err));

What is the output, and why?

for...of Loop:

const iterable = [1, 2, 3];
for (let value of iterable) {
  console.log(value);
}

What is the output, and why?

NaN and Equality:
```
console.log(NaN == NaN);
console.log(NaN === NaN);
```
What is the output for each line, and why?

Global Variables and window Object:

var globalVar = 5;
console.log(window.globalVar === globalVar);

What is the output, and why?

Closure and setTimeout:

for (var i = 0; i < 5; i++) {
  setTimeout(function() {
    console.log(i);
  }, 1000);
}

What is the output, and why?

Generator Functions:

function* generatorFunction() {
  yield 1;
  yield 2;
  yield 3;
}
const generator = generatorFunction();
console.log(generator.next().value);
console.log(generator.next().value);

What is the output, and why?

Array.prototype.slice:

const array = [1, 2, 3, 4, 5];
const newArray = array.slice(2);
console.log(newArray);

What is the output, and why?

null and typeof:
```
console.log(typeof null);
```
What is the output, and why?

Object.freeze:

const obj = { prop: 42 };
Object.freeze(obj);
obj.prop = 10;
console.log(obj.prop);

What is the output, and why?

Event Bubbling and Event Capturing:

<div id="parent">
  <button id="child">Click me</button>
</div>
<script>
  document.getElementById('parent').addEventListener('click', function() {
    console.log('Parent clicked');
  }, true);
  document.getElementById('child').addEventListener('click', function() {
    console.log('Child clicked');
  }, true);
</script>

What is the order of log messages when you click the button, and why?

Array.prototype.splice:

const array = [1, 2, 3, 4, 5];
const removed = array.splice(2, 2);
console.log(array, removed);

What is the output, and why?

Function Constructor:

const func = new Function('return 10 + 5');
console.log(func());

What is the output, and why?

Object Destructuring:

const { x, y } = { x: 10, y: 20 };
console.log(x, y);

What is the output, and why?

Array.prototype.filter:

const numbers = [1, 2, 3, 4, 5];
const filtered = numbers.filter(function(num) {
  return num % 2 === 0;
});
console.log(filtered);

What is the output, and why?

arguments Object and Arrow Function:

const sum = () => {
  console.log(arguments.length);
};
sum(1, 2, 3);

What is the output, and why?

Object.keys and Inherited Properties:

function Animal() {}
Animal.prototype.legs = 4;
const dog = new Animal();
dog.bark = function() { console.log('Woof!'); };
console.log(Object.keys(dog));

What is the output, and why?

Object.defineProperty:

const obj = {};
Object.defineProperty(obj, 'x', { value: 42, writable: false });
obj.x = 10;
console.log(obj.x);

What is the output, and why?

Unicode Escape Sequences:
```
console.log('\u0061');
console.log('\u{1F602}');
```
What is the output, and why?

Array.prototype.concat:

const arr1 = [1, 2, 3];
const arr2 = [4, 5, 6];
const result = arr1.concat(arr2);
console.log(result);

What is the output, and why?

JSON.parse with Reviver Function:

const jsonString = '{"name":"John","age":30,"city":"New York"}';
const obj = JSON.parse(jsonString, function(key, value) {
  if (key === 'age') return value + 5;
  return value;
});
console.log(obj);

What is the output, and why?

String.prototype.replace with Regular Expression:

const str = 'apple orange apple banana';
const result = str.replace(/apple/g, 'fruit');
console.log(result);

What is the output, and why?

Object.prototype.toString:

const obj = {};
console.log(Object.prototype.toString.call(obj));

What is the output, and why?

Object.entries:

const obj = { a: 1, b: 2, c: 3 };
const entries = Object.entries(obj);
console.log(entries);

What is the output, and why?

Array.prototype.map with thisArg:

const numbers = [1, 2, 3];
const doubled = numbers.map(function(num) {
  return num * this.multiplier;
}, { multiplier: 2 });
console.log(doubled);

What is the output, and why?

typeof Operator and Symbol:

const sym = Symbol('mySymbol');
console.log(typeof sym);

What is the output, and why?

Object.prototype.hasOwnProperty:

const obj = { a: 1, b: 2 };
console.log(obj.hasOwnProperty('a'));
console.log(obj.hasOwnProperty('toString'));

What is the output, and why?

Array.from:

const arrayLike = { 0: 'a', 1: 'b', length: 2 };
const newArray = Array.from(arrayLike);
console.log(newArray);

What is the output, and why?

RegExp Object and exec:

const pattern = /a/g;
const str = 'abcabc';
let match;
while ((match = pattern.exec(str)) !== null) {
  console.log(match[0], pattern.lastIndex);
}

What is the output, and why?

Object.setPrototypeOf:

const obj1 = { x: 10 };
const obj2 = { y: 20 };
Object.setPrototypeOf(obj2, obj1);
console.log(obj2.x);

What is the output, and why?

Promise.race:

const promise1 = new Promise(resolve => setTimeout(() => resolve('Promise 1'), 1000));
const promise2 = new Promise(resolve => setTimeout(() => resolve('Promise 2'), 500));
Promise.race([promise1, promise2])
  .then(value => console.log(value))
  .catch(err => console.error(err));

What is the output, and why?

Number and Decimal Representation:
```
console.log(0.1 + 0.2 === 0.3);
```
What is the output, and why?

Date Object and Month Index:

const date = new Date(2022, 0, 31);
console.log(date.getMonth());

What is the output, and why?

String.prototype.slice:

const str = 'abcdef';
const sliced = str.slice(-3, -1);
console.log(sliced);

What is the output, and why?

this in Arrow Functions and Constructor Functions:

function Person() {
  this.age = 0;
  setInterval(() => {
    this.age++;
    console.log(this.age);
  }, 1000);
}
const person = new Person();

What is the output, and why?

Array.prototype.every:

const numbers = [2, 4, 6, 8];
const allEven = numbers.every(function(num) {
  return num % 2 === 0;
});
console.log(allEven);

What is the output, and why?

Promise and async/await:

async function example() {
  return Promise.resolve('Hello');
}
example().then(value => console.log(value));

What is the output, and why?

Array.prototype.findIndex:

const numbers = [10, 20, 30, 40, 50];
const index = numbers.findIndex(function(num) {
  return num > 25;
});
console.log(index);

What is the output, and why?

for...in Loop and Object Properties:

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

What is the output, and why?

Object.create and Inheritance:

const parent = { x: 10 };
const child = Object.create(parent);
console.log(child.x);

What is the output, and why?

Array.prototype.some:

const numbers = [1, 3, 5, 7, 9];
const someEven = numbers.some(function(num) {
  return num % 2 === 0;
});
console.log(someEven);

What is the output, and why?

Promise and Promise.resolve:

const promise = Promise.resolve('Resolved');
promise.then(value => console.log(value));

What is the output, and why?

String.prototype.indexOf:

const str = 'hello world';
console.log(str.indexOf('o'));
console.log(str.indexOf('o', 5));

What is the output for each line, and why?

Array.isArray and instanceof:

const arr = [1, 2, 3];
console.log(Array.isArray(arr));
console.log(arr instanceof Array);

What is the output for each line, and why?

Date Object and Timezone Offset:

const date = new Date('2022-03-01T12:00:00Z');
console.log(date.toISOString());

What is the output, and why?

JSON.stringify with Replacer Function:

const obj = { a: 1, b: 2, c: 3 };
const jsonString = JSON.stringify(obj, function(key, value) {
  if (key === 'b') return undefined;
  return value;
});
console.log(jsonString);

What is the output, and why?

Array.prototype.reverse:

const arr = [1, 2, 3, 4, 5];
const reversed = arr.reverse();
console.log(reversed);

What is the output, and why?

RegExp and exec with Global Flag:

const pattern = /a/g;
const str = 'abcabc';
let match;
while ((match = pattern.exec(str)) !== null) {
  console.log(match[0], pattern.lastIndex);
}

What is the output, and why?

Number and toString:

const num = 42;
console.log(num.toString());
console.log((42).toString());

What is the output for each line, and why?

Object.keys and Property Order:

const obj = { 10: 'a', 2: 'b', 7: 'c' };
console.log(Object.keys(obj));

What is the output, and why?

Array.prototype.join:

const arr = [1, 2, 3];
const joined = arr.join('-');
console.log(joined);

What is the output, and why?

Math.max and apply:

const numbers = [1, 2, 3, 4, 5];
const max = Math.max.apply(null, numbers);
console.log(max);

What is the output, and why?

JSON.parse with Reviver Function:

const jsonString = '{"a":1,"b":2,"c":3}';
const obj = JSON.parse(jsonString, function(key, value) {
  if (key === 'b') return value * 10;
  return value;
});
console.log(obj);

What is the output, and why?

String.prototype.split:

const str = 'apple,orange,banana';
const splitArray = str.split(',');
console.log(splitArray);

What is the output, and why?

Object.defineProperty and Getter/Setter:

const obj = {};
let value = 42;
Object.defineProperty(obj, 'x', {
  get: function() { return value; },
  set: function(newValue) { value = newValue; }
});
obj.x = 10;
console.log(obj.x);

What is the output, and why?

Date Object and Day of the Week:

const date = new Date('2022-03-01T12:00:00Z');
console.log(date.getDay());

What is the output, and why?

99 JavaScript Tricky Questions | Must Have

Comments

More from this blog

Understanding Blocking and Non-Blocking Tasks in Node.js

Node.js Internals: Why Libuv and Worker Threads Are Fundamentally Different

NGINX configuration with docker

Node Cluster and Worker Thread Node.js

Learn everything about cookies in fastAPI

Command Palette

Comments

More from this blog