Validating 10,000-Row CSV Uploads in 20ms Before They Reach the Server

A developer describes how importing large billing CSV files in web apps often leads to slow uploads and repeated rejections from server-side errors discovered late. Checking for duplicate IDs with a nested array scan causes significant slowdowns on 10,000-row files, so the approach switches to a Set for constant-time lookups. The validator checks all columns on each row so users see every error at once rather than one per attempt. In a test with a 10,000-row CSV containing four planted errors, validation finished in about 20 milliseconds, avoiding an upload of roughly 419 KB. The article also raises the problem of storing large drafts in localStorage, which hits browser size limits, and points toward IndexedDB as an alternative.