Implement JavaScript functions like utility methods, polyfills, or DOM APIs.
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GUIDES
FlattenImplement a function that recursively flattens an array into a single level deep
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DebounceImplement a function that delays execution until calls have stopped for a given time
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Debounce IIPremiumImplement a debounce function that comes with a cancel method to cancel delayed invocations and a flush method to immediately invoke them
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useQueryImplement a hook that manages a promise resolution
ClassnamesImplement a function that conditionally joins CSS class names together
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Classnames IIPremiumImplement a function that conditionally joins CSS class names together and also handles de-duplication and function values
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useBooleanImplement a hook that manages a boolean state, with additional convenience utility methods
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useDefaultPremiumImplement a hook that returns the default value when state is null or undefined
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Promise.allImplement the Promise.all() function that resolves to an array of results if all the input elements are resolved or rejects otherwise
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ThrottleImplement a function that limits how often a function can run
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Map AsyncPremiumImplement a function that maps an array of items with an asynchronous mapping function
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Event EmitterImplement a class that can subscribe to and emit events that trigger attached callback functions
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getElementsByStyleImplement a function to get all DOM elements that are rendered using the specified style
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Promise.racePremiumImplement the Promise.race() function that resolves or rejects when any of the input elements are resolved or rejected
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Deep CloneImplement a function that performs a deep copy of a JSON-serializable value
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Promise.anyImplement the Promise.any() function that resolves when any of the input elements are resolved
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useArrayImplement a hook that manages an array of items
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useCounterImplement a hook that manages a counter state, with some additional convenience utility methods
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useCounter IIImplement an optimized version of the useCounter hook
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Cancellable IntervalPremiumImplement a function that acts like setInterval but returns a function to cancel the interval
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useCycleImplement a hook that cycles through a sequence of values
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Backbone ModelPremiumImplement a class resembling Backbone.Model which allows storing of attributes/values and responding to changes in specific attribute values
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List FormatImplement a function that formats a list of items into a single readable string
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Map Async LimitImplement a function that maps an array of items with an asynchronous mapping function while not exceeding the concurrency limit
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useStateWithResetPremiumImplement a hook that's similar to useState but with an additional reset function that resets the state to its initial value
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ComposePremiumImplement a function that takes multiple functions as arguments and returns a new function that applies those functions in reverse
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Deep EqualImplement a function that determines if two values are equal
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Data SelectionPremiumImplement a function to filter rows of data matching a specified requirement
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Type UtilitiesImplement utilities to determine primitive variable types in JavaScript
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Deep OmitImplement a function that removes specified keys and their corresponding values from an object, including nested objects or arrays
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useMapPremiumImplement a hook that manages a JavaScript map
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PromisifyImplement a function that takes a function following the common error-first callback style and returns a version that returns promises
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Promisify IIPremiumImplement a promisify function that allows the original function to override the return value
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jQuery.cssPremiumImplement a jQuery-like function that sets the style of a DOM element
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MemoizeImplement a function that returns a memoized version of a function which accepts a single argument
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Squash ObjectImplement a function that returns a new object after squashing the input object into a single level of depth
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Unsquash ObjectPremiumImplement a function that reconstructs a nested object from dot-delimited keys
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getElementsByClassNameImplement a function to get all DOM elements that contain the specified classes
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Table of ContentsPremiumImplement a function to construct a table of contents from an HTML document
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CurryImplement a function that converts a multi-argument function into a chain of unary calls
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HTML SerializerImplement a function to serialize an object into an HTML string with indentation
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JSON.stringifyImplement a function that converts a JavaScript value into a JSON string
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JSON.stringify IIPremiumImplement a function that converts a JavaScript value into a JSON string
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Promise MergePremiumImplement a function to merge the results of two promises into a single value
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useTimeoutImplement a hook that invokes a callback function after a specified delay
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useCountdownPremiumImplement a hook that manages a countdown
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ClampPremiumImplement a function that restricts a number to an inclusive range
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Function LengthPremiumImplement a function that returns the number of parameters expected by a function
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Make CounterImplement a function that accepts an integer value and returns a function that can be repeatedly called to return increasing values
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MeanImplement a function that finds the mean of the values inside an array
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Number of ArgumentsPremiumImplement a function that returns the number of arguments it was called with
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SleepPremiumImplement a function that pauses for a specified duration before resuming execution
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Array.prototype.squarePremiumImplement a custom Array.prototype.square() method that squares the values in an array
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Cancellable TimeoutPremiumImplement a function that acts like setTimeout but returns a function to cancel the pending callback
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ChunkPremiumImplement a function that creates an array of elements split into smaller groups of a specified size
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CompactPremiumImplement a function that creates an array with all falsy values removed
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DifferencePremiumImplement a function that returns the values from one array that are not present in another
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Drop Right WhilePremiumImplement a function that excludes elements from the end of an array until the predicate returns false
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Drop WhilePremiumImplement a function that excludes elements from the beginning of an array until the predicate returns false
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FillPremiumImplement a function that fills an array with values within specified indices
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Find IndexPremiumImplement a function that returns the index of the first element in the array that satisfies the provided testing function
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Find Last IndexPremiumImplement a function that returns the index of the last element in the array that satisfies the provided testing function
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From PairsPremiumImplement a function that returns an object composed from key-value pairs
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Function.prototype.applyPremiumImplement the Function.prototype.apply() function that calls the function with a given `this` value and arguments as an array
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Function.prototype.callImplement the Function.prototype.call() function that calls the function with a given `this` value and provided arguments
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GetPremiumImplement a function to safely access a nested value in an object
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In RangePremiumImplement a function to check if a number falls between two numbers
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IntersectionPremiumImplement a function that returns the unique values present in every array
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Max ByPremiumImplement a function that finds the maximum element based on the specified criteria
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Min ByImplement a function that finds the minimum element based on the specified criteria
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Object MapPremiumImplement a function to transform values within an object
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OncePremiumImplement a function that accepts a callback and restricts its invocation to at most once
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Promise.rejectPremiumImplement a function to return a Promise object rejected with a reason
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RangePremiumImplement a function that returns a sequence of numbers in ascending order
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Range RightPremiumImplement a function that returns a sequence of numbers in descending order
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SingletonPremiumImplement a Singleton class that ensures a class has only one instance while providing a global point of access to that instance
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Unique ArrayPremiumImplement a function to remove all duplicate values from an array
CycleImplement a function that takes one or more values and returns a function that cycles through those values each time it is called
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Function.prototype.bindPremiumImplement the Function.prototype.bind() function that creates a new function with the `this` keyword set to a provided value
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SizePremiumImplement a function that returns the size of a collection
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SumPremiumImplement 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
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Type Utilities IIImplement utilities to determine non-primitive variable types in JavaScript
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useBoolean IIPremiumImplement an optimized version of the useBoolean hook
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useClickAnywhereImplement a hook that handles click events anywhere in the window
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useEffectOncePremiumImplement a hook that runs an effect only once
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useFocusPremiumImplement a hook that enables programmatic focusing of an element
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usePreviousPremiumImplement a hook that returns the previous value of a state
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useTogglePremiumImplement a hook that manages a boolean toggle state
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Reading OrderImplement a function that sorts a 2D canvas elements in reading order
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Make Counter IIPremiumImplement a function that returns a counter object with methods to retrieve and manipulate the value
Count ByImplement a function that counts how many array elements map to each key
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Group ByPremiumImplement a function that groups array values based on a function
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Intersection ByPremiumImplement a function that returns an array of unique values that are included in all given arrays based on a provided iteratee function
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Intersection WithPremiumImplement a function that computes the intersection of arrays using a comparator function
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Is EmptyPremiumImplement a function to check whether a value should be treated as empty
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LimitPremiumImplement a function that accepts a callback and restricts its invocation to at most N times
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Promise.resolvePremiumImplement a function to resolve a given value to a Promise
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Promise TimeoutPremiumImplement a function that resolves a promise if it is fulfilled within a timeout period and rejects otherwise
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Promise.withResolversPremiumImplement a function that returns an object containing a new `Promise` object and two functions to resolve or reject it
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TurtlePremiumImplement a Turtle class that moves a turtle on a 2D plane
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Union ByPremiumImplement a function that creates an array of unique values, in order, from all given arrays.
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useBreakpointPremiumImplement a hook that returns the current breakpoint name based on the current window width
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useClickOutsidePremiumImplement a hook that detects clicks outside of a specified element
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useDebounceImplement a hook that debounces a value
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useEventListenerPremiumImplement a hook that subscribes to browser events
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useHoverPremiumImplement a hook that tracks whether an element is being hovered
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useIntervalPremiumImplement a hook that creates an interval that invokes a callback function at a specified delay
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useKeyPressPremiumImplement a hook that subscribes to keyboard events
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useObjectPremiumImplement a hook that manages an object value
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useSetImplement a hook that manages a JavaScript set
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useStepPremiumImplement a hook that manages a step counter for a multi-step process
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useThrottlePremiumImplement a hook that throttles a value
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useWindowSizeImplement a hook that returns the current height and width of the window
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Camel Case KeysPremiumImplement a function to convert all the keys in an object to camel case
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Compact IIPremiumImplement a function that returns an object with all falsey values removed
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Conforms ToPremiumImplement a function that checks if object conforms to source
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Console Log HistoryPremiumImplement a function that intercepts console.log() calls and records the arguments it was invoked with
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Curry IIPremiumImplement a function that transforms a function that takes multiple arguments into a function that can be repeatedly called with any number of arguments
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Data MergingImplement a function to merge rows of data from the same user
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Event Emitter IIPremiumImplement a class that can subscribe to and emit events that trigger attached callback functions. Subscription objects are returned and can unsubscribe itself
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getElementsByTagNamePremiumImplement a function to get all DOM elements that match a tag
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Identical DOM TreesPremiumImplement a function to determine if two DOM trees are the same
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jQuery Class ManipulationPremiumImplement a set of jQuery-like functions that manipulates classes on a DOM element
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Lowest Hiding ElementImplement a function to find the deepest DOM element that would hide a set of elements
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MiddlewaresImplement a function that composes asynchronous middleware functions like in Koa.js
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Node RegistryPremiumImplement a registry that stores values keyed by DOM nodes
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Promise.allSettledPremiumImplement the Promise.allSettled() function that resolves to an array of outcomes when all the input elements are either resolved or rejected
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Resumable IntervalPremiumImplement a function that creates a resumable interval object
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Text SearchPremiumImplement a function to highlight text if a searched term appears within it
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Undo / Redo ManagerImplement a class that manages value history with undo and redo functionality
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useIdlePremiumImplement a hook that detects user inactivity
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useInputControlImplement a hook that manages a controlled input value and tracks its dirty & touched state
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useMediaQueryImplement a hook that subscribes and responds to media query changes (e.g. screen size, resolution, orientation, etc.)
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useMediatedStateImplement a hook that is similar to useState, but supports a mediation process
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Deep MapPremiumImplement a function to recursively transform values
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SpreadsheetImplement a spreadsheet class with numeric cells and addition formulas
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Styled Text RangesImplement a function that slices styled text represented as text plus style ranges
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Test RunnerImplement a tiny test runner that registers tests, executes them, and summarizes the results
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Text Search IIPremiumImplement a function to highlight text if searched terms appear within it
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Deep MergePremiumImplement a function that merges two objects together
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Memoize IIPremiumImplement a function that returns a memoized version of a function which accepts any number of arguments
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Mini Object-relational MapperImplement a simplified Prisma-like in-memory Object-relational Mapper (ORM) with model delegates and basic CRUD operations
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Reading Order IIPremiumImplement a function that groups misaligned 2D canvas elements into rows before sorting
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Spreadsheet IIPremiumImplement a spreadsheet class with left-to-right arithmetic formulas
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Styled Text Ranges IIPremiumImplement a function that overwrites the style of a text range and normalizes the result
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Test Runner IIImplement a tiny test runner with nested suites and full spec names
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Test Runner IIIPremiumImplement a tiny test runner with inherited setup and cleanup hooks
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Undoable DatabasePremiumImplement a class that manages users with CRUD operations and undo/redo history
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Mini Object-relational Mapper IIPremiumImplement a simplified Prisma-like in-memory Object-relational Mapper (ORM) with richer filtering and sorting
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Curry IIIPremiumImplement a function which transforms a function which takes variadic arguments into a function that can be repeatedly called with any number of arguments