Reading a million-line file in a browser tab

Slice and stream

File.slice() gives cheap views into a file. A worker reads 16 MB at a time through a streaming TextDecoder, which carries partial multi-byte characters across slice boundaries — essential for Bengali, where most characters are three bytes in UTF-8.

Index offsets, not strings

The worker stores the byte offset of every line in chunked Uint32Arrays. Fetching lines 4,000,000–4,000,255 is then a single slice and decode.

Search cooperatively

Search runs in the worker, yields to its event loop regularly, streams progress, and cancels by bumping a generation counter — so typing a new query instantly abandons the old one.

Don't trust the scrollbar

Browsers cap element height (around 33 million pixels in Chromium). A spacer-based virtual list breaks long before a million lines at comfortable line heights. Tracking the position as an anchor line plus a pixel offset removes the ceiling — at the cost of drawing your own scrollbar.