Array map — JavaScript Challenge | FineFrontend

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JavaScript interview question

Array map

easy10 min

Implement arrayMap to recreate the core behavior of Array.prototype.map using a loop. Work with a standalone function that receives the array as its first argument.

Requirements

Accept an actual JavaScript array and a callback. Array-like objects, Array subclasses/species, and prototype installation are outside this beginner exercise.
Do not use native map, filter, reduce, reduceRight, flatMap, or forEach to implement the function. Do not modify Array.prototype.
Visit existing indexes in ascending order, skip sparse holes, and capture the initial length so appended items are not visited. Check each index when it is reached so deleted future entries are skipped.
Pass the value, index, and original array to the callback. Honor the optional thisArg for regular function callbacks.
Do not mutate the input array yourself. Callback side effects are permitted; propagate callback exceptions.
Throw TypeError when the callback is not a function, even when the array is empty.
Return a new array of the original length, preserving holes. Store every callback result, including undefined and nested arrays, without flattening.

Examples

Example 1

Input: arrayMap([1, 2, 3], value => value * 2)
Output: [2, 4, 6]

Example 2

Input: arrayMap([], value => value)
Output: []

arrayMap(array, callback, thisArg?) — standalone equivalent for arrays.

What the tests check

Transforms values in order
Supports strings and objects
Does not flatten callback results
Preserves holes but maps explicit undefined
Retains explicit undefined callback results
Passes the current value, index, and original array
Returns a new array and leaves input unchanged
Handles an empty array without callbacks
Supports callback thisArg
Ignores appended items and skips deleted future entries
Propagates callback failures
Rejects a non-function callback even for an empty array

References

Array.prototype.map

ArraysFunctionsCallbacksIterationSparse arrays

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