npm run local <directory-name> <output-filename>

e.g.:

npm run local testData output.dat

This will globally install the module, meaning you can subsequently run it using the command name:

merge_files testData output.dat

Updated: Traverse all the files at the same time, checking individual sort order as we go, and checking across the K files for the next lexicographically smallest line for each of the N lines in all files. O(NK).

Updated: Rewrote to use pointers to files and stream directly into the output file, so the memory usage is now just pointers to the files. O(K) where K is the total number of files.

Instead of finding the min item each time, store the top items as a min-heap linked to the file pointers, and delete min/insert next line on each interation. In a fibonacci heap, find-min and insert are O(1), whereas delete-min is O(lg N).

This would improve performance from O(KN) to O(K log N). If the number of files and number of lines per file grew at the same rate, this is equivalent to going from O(N^2) to O(N log N).