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JavaScript SQL database for browser and Node.js.
Handles both traditional relational tables and nested JSON data (NoSQL).
Export, store, and import data from localStorage, IndexedDB, or Excel.

In shell, you can do it in many ways:

cat input | xargs echo | tr ' ' ,

or

perl -pe 's/\r?\n/,/' input > output

I know you wanted this to be longer, but I don't really see the point of writing multi line script to solve such simple task - simple regexp (in case of perl solution) is fully workable, and it's not something artificially shortened - it's the type of code that I would use on daily basis to solve the issue at hand.

Miller is like awk, sed, cut, join, and sort for name-indexed data such as CSV, TSV, and tabular JSON. You get to work with your data using named fields, without needing to count positional column indices.

This is something the Unix toolkit always could have done, and arguably always should have done. It operates on key-value-pair data while the familiar Unix tools operate on integer-indexed fields: if the natural data structure for the latter is the array, then Miller’s natural data structure is the insertion-ordered hash map. This encompasses a variety of data formats, including but not limited to the familiar CSV, TSV, and JSON. (Miller can handle positionally-indexed data as a special case.)