Review Summary

I am requesting changes because there is a critical, blocking issue in your implementation that prevents the function from working as required. In src/formatDate.js, the call date.split(baseElement) uses baseElement without ever defining it (ESLint no-undef and a runtime ReferenceError), so the code cannot run. Per the task, the input separator must come from the fromFormat array, with its last element representing the separator; you should derive something like const inputSeparator = fromFormat[fromFormat.length - 1]; and then use date.split(inputSeparator) instead. Closely related, your current logic ignores this separator part of fromFormat and only uses fromFormat to find positions of DD, MM, and YY/ YYYY, which means the function does not fully honor the requirement that fromFormat describes both order and separator of the input date. Once you fix the undefined variable and properly use the fromFormat separator when splitting, your reordering and year conversion logic (including the YY ↔ YYYY rules) should align well with the specification. As a minor improvement, consider clearer names like inputSeparator and outputSeparator to make the intent of each step easier to follow. Your core idea is solid, so with these changes you’ll have a robust solution that matches the task requirements.

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Review Summary

I am requesting changes because there is a critical bug in how you derive the input separator from fromFormat, and this prevents the function from correctly parsing the date string for all specified examples. Right now, baseElement is taken as fromFormat[toFormat.length - 1], which can be a format token like 'DD' instead of the actual separator, so date.split(baseElement) will not work as required for valid inputs such as ['YYYY','MM','DD','-']. You need to change the separator derivation so it always uses the last element of fromFormat, for example const baseElement = fromFormat[fromFormat.length - 1];, and then keep using that to split the incoming date string [CHECKLIST ITEM #4, CHECKLIST ITEM #5]. Once that is fixed, your logic for reordering parts and converting between YY and YYYY appears to meet the task requirements. As a minor improvement, consider renaming baseElement and changeElement to something like inputSeparator and outputSeparator to make the intent clearer, though this is not blocking. Overall, you are very close; fixing the separator handling so it matches the fromFormat specification should make your solution correct and complete.

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Review Summary

I am approving your solution because your formatDate implementation correctly handles all required behaviors: it derives the input separator from the last element of fromFormat, splits the input date with it, and uses the last element of toFormat to join the output string, matching the separator rules [CHECKLIST ITEM #4 and #5]. You accurately locate the day, month, and year positions based on fromFormat, reorder them for toFormat, and implement the YYYY ↔ YY conversions with the correct 20YY/19YY rule, which aligns with all specified examples. Your code focuses on the core logic without adding unnecessary validation, which is appropriate given the assumption that inputs are always well-formed.

One minor improvement opportunity is naming clarity—for example, a name like yearIndex instead of fromYearIndex could make the role of the variable more immediately obvious, but this is stylistic and your current naming is still understandable. Overall, this is solid, clean work that shows you carefully followed the requirements and previous feedback. Nice job implementing the rules precisely and keeping the solution straightforward and readable.

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