Fast JSON Viewer

Fast JSON Viewer is a free online JSON viewer for large and huge JSON files. It is designed for speed, privacy, and security: your file is parsed locally in your browser, with no server upload and no server-side parsing.

Use it to open, validate, search, and navigate massive JSON files with streaming parsing and virtual scrolling. The viewer is built for multi-gigabyte JSON files and has been tested with 10GB JSON files.

{
  "app"     :  "fast json viewer",
  "price"   :  "free",
  "privacy" :  "secure: no upload, no server parsing",
  "scale"   :  "huge json files — 10 GB tested",
  "workers" :  8,
  "engine"  :  "streaming parser + virtual scroll"
}

DRAG & DROP or click to open a .json file

total: — copy instrumentation

drop a file to visualize the instrumentation waterfall

Web Workers 8 — applied on next file load

Implementation JS WASM — applied on next file load

drop a file to see statistics

Worker	Bytes	Chars	ms	MB/s
—

Parallel Chunk Parsing

The file is split into N equal chunks, one per web worker. Each worker streams its chunk in 256 KB blocks, running a branchless byte-level parser across 23 boundary scenarios. Results are stitched on the main thread by selecting the scenario that matches the prior chunk's exit state.

Virtual Scroll Viewer

A sparse line → byte-offset index is built during parsing. The viewer slices only the bytes needed for the visible viewport — no full file is ever loaded into memory. First-page rendering begins immediately, before parsing finishes.

UTF-8 Chunk Boundaries

Before dispatching workers, the main thread sniffs each chunk boundary to ensure it does not split a multi-byte UTF-8 sequence. Boundaries are nudged backward by up to 3 bytes as needed.

Safari Mode

Safari has tighter per-tab memory limits. When detected, the parser uses 64 KB blocks and dispatches workers two at a time with GC pauses between batches.

JSON.VIEWER — the fastest in-browser JSON viewer

UI aesthetic inspired by sacred.computer / SRCL by internet-development

Parser — custom byte-level JSON chunk analyzer (23 boundary scenarios)
Viewer — virtual-scroll renderer with on-demand byte-range loading
Tests — Playwright E2E suite (~54 cases from RFC 8259 corpus)

▶

IDLE