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200+ JavaScript and TypeScript interview questions, from implementing common library APIs, utility functions, algorithms, to building UI components and more.

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Array.prototype.concatImplement the Array.prototype.concat() method
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Array.prototype.filterImplement the Array.prototype.filter() method
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Questions List

  • CounterWarm Up QuestionBuild a simple counter that increments whenever a button is clicked
    Available frameworks
  • Make CounterImplement a function that accepts an integer value and returns a function that can be repeatedly called to return increasing values
    Languages
    JSTS
  • New
    MeanImplement a function that finds the mean of the values inside an array
    Languages
    JSTS
  • New
    Min ByImplement a function that finds the minimum element based on the specified criteria
    Languages
    JSTS
  • StackImplement a stack data structure containing the common stack methods
    Languages
    JSTS
  • Type UtilitiesImplement utilities to determine primitive variable types in JavaScript
    Languages
    JSTS
  • CycleImplement a function that takes one or more values and returns a function that cycles through those values each time it is called
    Languages
    JSTS
  • Insertion SortImplement a function that performs an insertion sort
    Languages
    JSTS
  • Type Utilities IIImplement utilities to determine non-primitive variable types in JavaScript
    Languages
    JSTS
  • Mortgage CalculatorBuild a calculator that computes the monthly mortgage for a loan
    Available frameworks
  • Binary SearchImplement a function that performs binary search on an array of numbers
    Languages
    JSTS
  • Breadth-first SearchImplement a breadth-first search algorithm that traverses a directed graph in a breadth-first manner
    Languages
    JSTS
  • Count ByImplement a function that counts the number of times a value appears in an array based on a function or property name
    Languages
    JSTS
  • DebounceImplement a function to limit how many times a function can be executed by delaying the execution of the function until after a specified time after its last execution attempt
    Languages
    JSTS
  • Depth-first SearchImplement a depth-first search algorithm that traverses a directed graph in a depth-first manner
    Languages
    JSTS
  • Heap SortImplement a function that performs a heap sort
    Languages
    JSTS
  • QueueImplement a queue data structure containing the common queue methods
    Languages
    JSTS
  • TabsBuild a tabs component that displays a list of tab elements and one associated panel of content at a time
    Available frameworks
  • ClassnamesImplement a function that conditionally joins CSS class names together
    Languages
    CSSJSTS
  • Event EmitterImplement a class that can subscribe to and emit events that trigger attached callback functions
    Languages
    JSTS
  • FlattenImplement a function that recursively flattens an array into a single level deep
    Languages
    JSTS
  • Merge SortImplement a function that performs a recursive merge sort
    Languages
    JSTS
  • Promise.allImplement the Promise.all() function that resolves to an array of results if all the input elements are resolved or rejects otherwise
    Languages
    JSTS
  • Quick SortImplement a function that performs a recursive quick sort
    Languages
    JSTS
  • Todo ListBuild a Todo list that lets users add new tasks and delete existing tasks
    Available frameworks
  • Topological SortImplement a function that performs a topological sort
    Languages
    JSTS
  • Traffic LightBuild a traffic light where the lights switch from green to yellow to red after predetermined intervals and loop indefinitely
    Available frameworks
  • Deep CloneImplement a function that performs a deep copy of a value
    Languages
    JSTS
  • Digital ClockBuild a 7-segment digital clock that shows the current time
    Available frameworks
  • Job BoardBuild a job board that displays the latest job postings from Hacker News
    Available frameworks
  • ClampImplement a function to clamp a number within the inclusive lower and upper bounds
    Languages
    JSTS
  • Function LengthImplement a function that returns the number of parameters expected by a function
    Languages
    JSTS
  • Number of ArgumentsImplement a function that returns the number of arguments it was called with
    Languages
    JSTS
  • SleepImplement a function that pauses for a specified duration before resuming execution
    Languages
    JSTS
  • Array.prototype.squareImplement a custom Array.prototype.square() method that squares the values in an array
    Languages
    JSTS
  • Cancellable IntervalImplement a function that acts like setInterval but returns a function to cancel the interval
    Languages
    JSTS
  • Cancellable TimeoutImplement a function that acts like setTimeout but returns a function to cancel the pending callback
    Languages
    JSTS
  • ChunkImplement a function that creates an array of elements split into smaller groups of a specified size
    Languages
    JSTS
  • CompactImplement a function that creates an array with all falsey values removed
    Languages
    JSTS
  • DifferenceImplement a function that finds the difference in values between arrays
    Languages
    JSTS
  • Drop Right WhileImplement a function that excludes elements from the end of an array until the predicate returns false
    Languages
    JSTS
  • Drop WhileImplement a function that excludes elements from the beginning of an array until the predicate returns false
    Languages
    JSTS
  • FillImplement a function that fills an array with values within specified indices
    Languages
    JSTS
  • Find IndexImplement a function that returns the index of the first element in the array that satisfies the provided testing function
    Languages
    JSTS
  • Find Last IndexImplement a function that returns the index of the last element in the array that satisfies the provided testing function
    Languages
    JSTS
  • From PairsImplement a function that returns an object composed from key-value pairs
    Languages
    JSTS
  • Function.prototype.applyImplement the Function.prototype.apply() function that calls the function with a given `this` value and arguments as an array
    Languages
    JSTS
  • Function.prototype.callImplement the Function.prototype.call() function that calls the function with a given `this` value and provided arguments
    Languages
    JSTS
  • GetImplement a function to safely access deeply-nested properties in JavaScript objects
    Languages
    JSTS
  • In RangeImplement a function to check if a number falls between two numbers
    Languages
    JSTS
  • IntersectionImplement a function that computes the intersection of arrays, returning a new array containing unique values present in all given arrays
    Languages
    JSTS
  • New
    Max ByImplement a function that finds the maximum element based on the specified criteria
    Languages
    JSTS
  • Object MapImplement a function to transform values within an object
    Languages
    JSTS
  • OnceImplement a function that accepts a callback and restricts its invocation to at most once
    Languages
    JSTS
  • Promise.rejectImplement a function to return a Promise object rejected with a reason
    Languages
    JSTS
  • New
    RangeImplement a function that returns a sequence of numbers in ascending order
    Languages
    JSTS
  • New
    Range RightImplement a function that returns a sequence of numbers in descending order
    Languages
    JSTS
  • Selection SortImplement a function that performs a selection sort
    Languages
    JSTS
  • SingletonImplement a Singleton class that ensures a class has only one instance while providing a global point of access to that instance
    Languages
    JSTS
  • New
    UnionImplement a function that creates an array of unique values, in order, from all given arrays
    Languages
    JSTS
  • Unique ArrayImplement a function to remove all duplicate values from an array
    Languages
    JSTS
  • New
    WithoutImplement a function that creates an array excluding all given values.
    Languages
    JSTS
  • AccordionBuild an accordion component that a displays a list of vertically stacked sections with each containing a title and content snippet
    Available frameworks
  • Array.prototype.atImplement the Array.prototype.at() method
    Languages
    JSTS
  • Array.prototype.filterImplement the Array.prototype.filter() method
    Languages
    JSTS
  • Array.prototype.mapImplement the Array.prototype.map() method
    Languages
    JSTS
  • Array.prototype.reduceImplement the Array.prototype.reduce() method
    Languages
    JSTS
  • ComposeImplement a function that takes multiple functions as arguments and returns a new function that applies those functions in reverse
    Languages
    JSTS
  • Flight BookerBuild a component that books a flight for specified dates
    Available frameworks
  • Function.prototype.bindImplement the Function.prototype.bind() function that creates a new function with the `this` keyword set to a provided value
    Languages
    JSTS
  • Generate TableGenerate a table of numbers given the rows and columns
    Available frameworks
  • jQuery.cssImplement a jQuery-like function that sets the style of a DOM element
    Languages
    HTMLCSSJSTS
  • New
    Mean ByImplement a function that finds the mean value based on the specified criteria
    Languages
    JSTS
  • Progress BarBuild a progress bar component that shows the percentage completion of an operation
    Available frameworks
  • Progress BarsBuild a list of progress bars that fill up gradually when they are added to the page
    Available frameworks
  • Promise.raceImplement the Promise.race() function that resolves or rejects when any of the input elements are resolved or rejected
    Languages
    JSTS
  • SizeImplement a function that returns the size of collection
    Languages
    JSTS
  • SumImplement a function that sums numbers by accepting a number and allows for repeated calling with more numbers until it is not called with any number
    Languages
    JSTS
  • New
    Sum ByImplement a function that finds the sum based on the specified criteria
    Languages
    JSTS
  • Temperature ConverterBuild a temperature converter widget that converts temperature values between Celsius and Fahrenheit
    Available frameworks
  • Make Counter IIImplement a function that returns a counter object with methods to retrieve and manipulate the value
    Languages
    JSTS
  • Array.prototype.concatImplement the Array.prototype.concat() method
    Languages
    JSTS
  • CurryImplement a function that transforms a function that takes multiple arguments into a function that can be repeatedly called with only one argument at a time
    Languages
    JSTS
  • Group ByImplement a function that groups values in an array based on a function or property name
    Languages
    JSTS
  • Intersection ByImplement a function that returns an array of unique values that are included in all given arrays based on a provided iteratee function
    Languages
    JSTS
  • Intersection WithComputes the intersection of arrays using a custom comparator function to determine equality between elements
    Languages
    JSTS
  • Is EmptyImplement a function to check if a value is an empty object, collection, map, or set
    Languages
    JSTS
  • LimitImplement a function that accepts a callback and restricts its invocation to at most N times
    Languages
    JSTS
  • Promise MergeImplement a function to merge the results of two promises into a single value
    Languages
    JSTS
  • Promise TimeoutImplement a function that resolves a promise if it is fulfilled within a timeout period and rejects otherwise
    Languages
    JSTS
  • Promise.resolveImplement a function to resolve a given value to a Promise
    Languages
    JSTS
  • PromisifyImplement a function that takes a function following the common error-first callback style and returns a version that returns promises
    Languages
    JSTS
  • Promisify IIImplement a promisify function that allows the original function to override the return value
    Languages
    JSTS
  • ThrottleImplement a function to control the execution of a function by limiting how many times it can execute over time
    Languages
    JSTS
  • TurtleImplement a Turtle class that moves a turtle on a 2D plane
    Languages
    JSTS
  • New
    Union ByImplement a function that creates an array of unique values, in order, from all given arrays.
    Languages
    JSTS
  • New
    Union WithImplement a function that creates an array of unique values, in order, from all given arrays, based on a specific comparison criteria
    Languages
    JSTS
  • New
    Unique ByImplement a function to remove all duplicate values from an array based on a specific criteria
    Languages
    JSTS
  • New
    Unique WithImplement a function to remove all duplicate values from an array based on a specific comparison criteria
    Languages
    JSTS
  • Accordion IIBuild an accessible accordion component that has the right ARIA roles, states, and properties
    Available frameworks
  • Accordion IIIBuild a fully accessible accordion component that has keyboard support according to ARIA specifications
    Available frameworks
  • Analog ClockBuild an analog clock where the hands update and move like a real clock
    Available frameworks
  • Camel Case KeysImplement a function to convert all the keys in an object to camel case
    Languages
    JSTS
  • Compact IIImplement a function that returns an object with all falsey values removed
    Languages
    JSTS
  • Curry IIImplement a function that transforms a function that takes multiple arguments into a function that can be repeatedly called with any number of arguments
    Languages
    JSTS
  • Data MergingImplement a function to merge rows of data from the same user
    Languages
    JSTS
  • Debounce IIImplement a debounce function that comes with a cancel method to cancel delayed invocations and a flush method to immediately invoke them
    Languages
    JSTS
  • Event Emitter IIImplement a class that can subscribe to and emit events that trigger attached callback functions. Subscription objects are returned and can unsubscribe itself
    Languages
    JSTS
  • getElementsByTagNameImplement a function to get all DOM elements that match a tag
    Languages
    HTMLJSTS
  • Grid LightsBuild a grid of lights where the lights deactivate in the reverse order they were activated
    Available frameworks
  • HTML SerializerImplement a function to serialize an object into an HTML string with indentation
    Languages
    HTMLJSTS
  • Identical DOM TreesImplement a function to determine if two DOM trees are the same
    Languages
    HTMLJSTS
  • jQuery Class ManipulationImplement a set of jQuery-like functions that manipulates classes on a DOM element
    Languages
    HTMLCSSJSTS
  • JSON.stringifyImplement a function that converts a JavaScript value into a JSON string
    Languages
    JSTS
  • Like ButtonBuild a Like button that changes appearance based on the states
    Available frameworks
  • List FormatImplement a function that formats a list of items into a single readable string
    Languages
    JSTS
  • Map AsyncImplement a function that maps an array of items with an asynchronous mapping function
    Languages
    JSTS
  • MemoizeImplement a function that returns a memoized version of a function which accepts a single argument
    Languages
    JSTS
  • Progress Bars IIBuild a list of progress bars that fill up gradually in sequence, one at a time
    Available frameworks
  • Promise.allSettledImplement the Promise.allSettled() function that resolves to an array of outcomes when all the input elements are either resolved or rejected
    Languages
    JSTS
  • Promise.anyImplement the Promise.any() function that resolves when any of the input elements are resolved
    Languages
    JSTS
  • Resumable IntervalImplement a function that creates a resumable interval object
    Languages
    JSTS
  • Squash ObjectImplement a function that returns a new object after squashing the input object into a single level of depth
    Languages
    JSTS
  • Star RatingBuild a star rating component that shows a row of star icons for users to select the number of filled stars corresponding to the rating
    Available frameworks
  • Tabs IIBuild a semi-accessible tabs component that has the right ARIA roles, states, and properties
    Available frameworks
  • Tabs IIIBuild a fully accessible tabs component that has keyboard support according to ARIA specifications
    Available frameworks
  • Text SearchImplement a function to highlight text if a searched term appears within it
    Languages
    HTMLJSTS
  • Deep EqualImplement a function that determines if two values are equal
    Languages
    JSTS
  • Deep MapImplement a function to recursively transform values
    Languages
    JSTS
  • getElementsByClassNameImplement a function to get all DOM elements that contain the specified classes
    Languages
    HTMLCSSJSTS
  • Map Async LimitImplement a function that maps an array of items with an asynchronous mapping function while not exceeding the concurrency limit
    Languages
    JSTS
  • Progress Bars IIIBuild a list of progress bars that fill up gradually concurrently, up to a limit of 3
    Available frameworks
  • Text Search IIImplement a function to highlight text if searched terms appear within it
    Languages
    HTMLJSTS
  • Tic-tac-toeBuild a tic-tac-toe game that is playable by two players
    Available frameworks
  • Deep MergeImplement a function that merges two objects together
    Languages
    JSTS
  • Deep OmitImplement a function that removes specified keys and their corresponding values from an object, including nested objects or arrays
    Languages
    JSTS
  • Memoize IIImplement a function that returns a memoized version of a function which accepts any number of arguments
    Languages
    JSTS
  • Signup FormBuild a signup form that does validation on user details and submits to a back end API
    Available frameworks
  • StopwatchBuild a stopwatch widget that can measure how much time has passed
    Available frameworks
  • Transfer ListBuild a component that allows transferring of items between two lists
    Available frameworks
  • Undoable CounterBuild a counter with a history of the values and ability to undo/redo actions
    Available frameworks
  • Whack-A-MoleBuild a popular arcade game where players attempt to hit moles as they pop up from holes in a board
    Available frameworks
  • Memory GameBuild a memory game where the player needs to match pairs of cards
    Available frameworks
  • Curry IIIImplement a function which transforms a function which takes variadic arguments into a function that can be repeatedly called with any number of arguments
    Languages
    JSTS
  • Classnames IIImplement a function that conditionally joins CSS class names together and also handles de-duplication and function values
    Languages
    CSSJSTS
  • Progress Bars IVBuild a list of progress bars that fill up gradually concurrently, up to a limit of 3 and allows for pausing and resuming
    Available frameworks
  • Backbone ModelImplement a class resembling Backbone.Model which allows storing of attributes/values and responding to changes in specific attribute values
    Languages
    JSTS
  • Data SelectionImplement a function to filter rows of data matching a specified requirement
    Languages
    JSTS
  • Table of ContentsImplement a function to construct a table of contents from an HTML document
    Languages
    HTMLJSTS
  • Tic-tac-toe IIBuild an N x N tic-tac-toe game that requires M consecutive marks to win
    Available frameworks
  • Transfer List IIBuild a component that allows transferring of items between two lists, bulk selection/unselection of items, and adding of new items
    Available frameworks
  • Deep Clone IIImplement a function that performs a deep copy of a value, but also handles circular references
    Languages
    JSTS
  • JSON.stringify IIImplement a function that converts a JavaScript value into a JSON string
    Languages
    JSTS
  • WordleBuild Wordle, the word-guessing game that took the world by storm
    Available frameworks