Javascript Coding Question

13 May 2025

Collection of Javascript Coding Interview Questions

Capitalize Every Word Task: Given an array of lowercase words, return a new array where every word starts with a capital letter.

Input let arr = [“apple”, “banana”, “apple”, “orange”, “banana”, “apple”];

Output [“Apple”, “Banana”, “Apple”, “Orange”, “Banana”, “Apple”];
Add a New Property Task: Create a new array where each object has an additional Boolean property isAdult based on whether the age is 18 or above.

Input let arr = [ { name: “a”, age: 50 }, { name: “b”, age: 15 }, { name: “c”, age: 30 }, { name: “e”, age: 18 }, { name: “e”, age: 7 }, ];
Extract names Task: Return only the names from an array of objects containing various properties.

Input let arr = [ { name: “a”, age: 50 }, { name: “b”, age: 15 }, { name: “c”, age: 30 }, { name: “e”, age: 18 }, { name: “e”, age: 7 }, ];

Output [‘a’, ‘b’, ‘c’, ‘e’, ‘e’]
Convert Temperature Task: Convert an array of temperatures in Celsius to Fahrenheit using .map().
Create HTML Elements Task: Generate strings formatted as HTML <li> elements from an array of product names.
Find Total Price Task: Calculate the total price for items in a shopping cart, based on price and quantity.

Input let arr = [ { item: “Apple”, price: 0.5, quantity: 4 }, { item: “Banana”, price: 0.3, quantity: 6 }, { item: “Cherry”, price: 2.0, quantity: 1 }, { item: “Apple”, price: 0.5, quantity: 4 }, { item: “Banana”, price: 0.3, quantity: 6 }, { item: “Cherry”, price: 2.0, quantity: 1 }, { item: “Apple”, price: 0.5, quantity: 4 }, { item: “Banana”, price: 0.3, quantity: 6 }, { item: “Cherry”, price: 2.0, quantity: 1 }, { item: “Apple”, price: 0.5, quantity: 4 }, { item: “Banana”, price: 0.3, quantity: 6 }, { item: “Cherry”, price: 2.0, quantity: 1 }, ];
Group by Category Task: Organize products into categories by grouping them based on their category field.

Input let arr = [ { name: “Shirt”, category: “Clothing” }, { name: “Pants”, category: “Clothing” }, { name: “Apple”, category: “Fruit” }, { name: “Banana”, category: “Fruit” }, { name: “TV”, category: “Electronics” }, ];
Count Frequency Task: Count how frequently each item appears in an array.

Input let arr = [“apple”, “banana”, “apple”, “orange”, “banana”, “apple”];

Output {apple: 3, banana: 2, orange: 1, guvava: 1}
Find the Longest String Task: Return the longest string from a given array of strings.

Input const strings = [“cat”, “elephant”, “dog”, “giraffe”, “hippopotamus”, “goat”];

Output hippopotamus
Merge Arrays of Objects by ID Task: Use .reduce() to merge two arrays of objects by their id fields, summing up values for duplicate ids.

Input let array1 = [ { id: 1, value: 10 }, { id: 2, value: 20 }, { id: 3, value: 30 } ];

Input let array2 = [ { id: 2, value: 15 }, { id: 3, value: 25 }, { id: 4, value: 35 } ]

Output [ { id: 1, value: 10 }, // From array1 (unique) { id: 2, value: 35 }, // Sum of id: 2 from array1 (20) and array2 (15) { id: 3, value: 55 }, // Sum of id: 3 from array1 (30) and array2 (25) { id: 4, value: 35 } // From array2 (unique) ] 11. Square the Numbers
Task: Return a new array where each number is squared.

Input Example Input: [2, 3, 4, 5]

Output [4, 9, 16, 25]
Format Prices Task: Convert an array of prices in cents to dollar-formatted strings.

Input [199, 2500, 320]

Output [“$1.99”, “$25.00”, “$3.20”]
Add Timestamps Task: Add a timestamp to each object in an array of logs.

Input [{ action: “login” }, { action: “logout” }]
Sum All Scores Task: Calculate the total score from an array of player objects.

Input [{ name: “A”, score: 5 }, { name: “B”, score: 8 }, { name: “C”, score: 12 }]

Output 25
Create Frequency Map Task: Count the occurrences of each value in an array.

Input [“red”, “blue”, “red”, “green”, “blue”, “blue”]

Output { red: 2, blue: 3, green: 1 }
Filter Even Numbers Task: Return only even numbers from the input array.

Input [1, 2, 3, 4, 5, 6]

Output [2, 4, 6]
Remove Null or Undefined Values Task: Return a clean array excluding any null or undefined values.

Input [10, null, 20, undefined, 30]

Output [10, 20, 30]
Normalize and Filter Usernames Task: Trim and convert usernames to lowercase, then filter out names shorter than 4 characters.

Input [” Bob”, “ALICE “, “Jo “, “ Charlie “, “eve “]

Output [“alice”, “charlie”]
Aggregate Expenses by Category Task: Group expenses by category and calculate the total amount per category.

Input [ { category: “Food”, amount: 20 }, { category: “Transport”, amount: 15 }, { category: “Food”, amount: 30 } ]

Output { Food: 50, Transport: 15 }
Filter Out Even Numbers Task: From a list of numbers, return only the odd numbers.

Input [1, 2, 3, 4, 5, 6, 7]

Output[1, 3, 5, 7]
Remove Falsy Values Task: Remove all falsy values (false, null, 0, “”, undefined, NaN) from an array.

Input [0, 1, false, 2, ‘’, 3, null, undefined]

Output [1, 2, 3]
Filter Active Users Task: From a list of user objects, return only users who are active.

Input [ { name: “Alice”, active: true }, { name: “Bob”, active: false }, { name: “Charlie”, active: true } ]

Output Objects for Alice and Charlie.
Find Items in Stock Task: Given an array of products, return only those where stock > 0.

Input [{ name: “Pen”, stock: 10 }, { name: “Pencil”, stock: 0 }, { name: “Notebook”, stock: 5 }]

Output Pen and Notebook.
Filter by Minimum Score Task: From an array of students with test scores, return those who scored 60 or higher.

Input [{ name: “Alex”, score: 45 }, { name: “Bella”, score: 75 }, { name: “Chris”, score: 60 }]

Output Bella and Chris.
Emails from Specific Domain Task: Filter users whose email ends with @example.com.

Input [{ email: “john@example.com” }, { email: “jane@gmail.com” }, { email: “doe@example.com” }]

Output John and Doe.
Remove Duplicate IDs Task: From a list of items with IDs, remove duplicates (assume duplicates have the same ID).

Input [{ id: 1 }, { id: 2 }, { id: 1 }, { id: 3 }]

Output: Only one of each unique ID.
Filter by String Length Task: Return only strings longer than 5 characters.

Input [“apple”, “banana”, “kiwi”, “strawberry”]

Output [“banana”, “strawberry”]
Get Valid Dates Task: Filter an array to remove invalid date strings.

Input [“2023-10-10”, “not-a-date”, “2022-01-01”, “12345”]

Output Only valid date strings that can be parsed.
Adults Only Task: From an array of people, return only those aged 18 or older.

Input [ { name: “Tom”, age: 17 }, { name: “Lucy”, age: 22 }, { name: “Mark”, age: 18 }]

Output Lucy and Mark.
Flatten, Filter, and Square Task: Given a nested array of numbers, flatten it, remove odd numbers, and return their squares.

Input [[1, 2], [3, 4], [5]]

Output Flatten, Filter evens, Square, Expected Output: [4, 16]
Comment Stats Task: Given an array of post objects, each with a comments array, calculate: Total number of comments Names of users who commented (deduplicated)

Input [{ id: 1, comments: [{ user: “Alice” }, { user: “Bob” }] }, { id: 2, comments: [{ user: “Bob” }, { user: “Charlie” }] }]

Output { totalComments: 4, users: [“Alice”, “Bob”, “Charlie”] }
Input let arr = [ { item: “Apple”, price: 0.5, quantity: 4 }, { item: “Banana”, price: 0.3, quantity: 6 }, { item: “Cherry”, price: 2.0, quantity: 1 }, { item: “Apple”, price: 0.5, quantity: 4 }, { item: “Banana”, price: 0.3, quantity: 6 }, { item: “Cherry”, price: 2.0, quantity: 1 }, { item: “Apple”, price: 0.5, quantity: 4 }, { item: “Banana”, price: 0.3, quantity: 6 }, { item: “Cherry”, price: 2.0, quantity: 1 }, { item: “Apple”, price: 0.5, quantity: 4 }, { item: “Banana”, price: 0.3, quantity: 6 }, { item: “Cherry”, price: 2.0, quantity: 1 }, ];

Output apple: {item:apple , price: .5, quantity:8, tota: 4}
What is the difference between exec () and test () methods in javascript?
What is the output of the following code?

const b = [1, 2, 3, 4, 5, 6, 7, 8, 9, 10];

for (let i = 0; i < 10; i++) { setTimeout(() => console.log(b[i]), 1000); } for (var i = 0; i < 10; i++) { setTimeout(() => console.log(b[i]), 1000);
Write a function that performs binary search on a sorted array

function binarySearch(arr,value,startPos,endPos){ if(startPos > endPos) return -1;

let middleIndex = Math.floor(startPos+endPos)/2;

if(arr[middleIndex] === value) return middleIndex;

elsif(arr[middleIndex] > value){ return binarySearch(arr,value,startPos,middleIndex-1); } else{ return binarySearch(arr,value,middleIndex+1,endPos); } }
Build User Lookup by ID Task: Convert an array of users into an object where the key is the user ID, and the value is the full user object.

Input [ { id: 101, name: “A” }, { id: 102, name: “B” } ]

Output { 101: { id: 101, name: “A” }, 102: { id: 102, name: “B” } }
Filter & Format Available Inventory Task: From a list of inventory items, return a new array of available items (in stock), formatted as strings.

Input [{ item: “Shoes”, stock: 5 }, { item: “Socks”, stock: 0 }, { item: “Jacket”, stock: 3 }]

Output [“Shoes (5 left)”, “Jacket (3 left)”]
Filter Unique Users by Email Task: Given a list of user objects with email, filter out duplicates (keep only the first occurrence of each unique email).

Input [{ name: “A”, email: “a@mail.com” }, { name: “B”, email: “b@mail.com” }, { name: “A2”, email: “a@mail.com” } ]

Output First two objects (3rd one filtered out).
Complex Invoice Summary Task: Given invoices with line items, calculate: Total for each invoice Filter invoices where total > 100

Input [ { id: 1, items: [{ price: 30 }, { price: 40 }] }, { id: 2, items: [{ price: 80 }, { price: 50 }] } ]

Output [{ id: 2, total: 130 }]
Find Most Frequent Element Task: From an array of strings, return the element that appears the most.

Input [“apple”, “banana”, “apple”, “orange”, “banana”, “apple”]

Output “apple”
Filter Objects Based on a Nested Property Task: Filter objects in an array based on the value of a nested property.

Input const posts = [ { id: 1, meta: { published: true } }, { id: 2, meta: { published: false } } ];

Output Only the objects where meta.published is true.
Explain Code Behaviour Task: Understand what the following code does:

Input const result = arr.filter((val, index, self) => self.indexOf(val) === index);

Explanation Filters unique elements in the array. self.indexOf(val) finds the first occurrence of the value. Keeps the value only if its first occurrence index matches the current index.
Filter Strings Starting with a Capital Letter Task: Filter an array of strings to only include those starting with a capital letter.

Input [“apple”, “Banana”, “kiwi”, “Strawberry”]

Output [“Banana”, “Strawberry”]
Write a Polyfill for Array.prototype.filter() Task: Implement a custom version of the .filter() method that mimics its functionality. Create a function that takes a callback and applies it to each element of an array.
Result of Filtering an Empty Array Task: Understand the behavior when .filter() is applied to an empty array.

Output Always returns an empty array, since there are no elements to iterate over.
Optimize .filter() for Large Datasets Task: Discuss techniques for efficiently using .filter() on large datasets. Combine .filter() with pagination or chunking to process smaller subsets.
Async/Await in a .filter() Callback Task: Can .filter() support asynchronous operations inside its callback?
Filter Common Elements in Two Arrays Task: From two arrays, return elements that are present in both.

Input const a = [1, 2, 3, 4]; const b = [2, 4];

Output [2, 4]
Result of the Following Code Task: Predict the output and explain behavior:

Input const res = [1, 2, 3].filter((x, i, arr) => { arr.pop(); return true; });
Given two strings str1 and str2, determine if str2 is an anagram of str1 and return true if it is, and false otherwise.