A viewer is not a formatter with a nicer background. It has to open a file that is too big to read, let you find one value inside a million, and tell you the shape of the document before you have scrolled it. These four do that for the file types that carry JSON.
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File viewers
Open a file and read its structure, without waiting for an editor to load the whole thing.
Why a text editor is the wrong tool above about 50 MB
Most editors build a syntax-highlighted view of every character in the file, so memory use scales with file size rather than with what is on screen. The viewers here index the document once into a flat node array and then paint only the rows inside the viewport, which is why scrolling stays smooth on documents with hundreds of thousands of nodes. The limit that remains is the browser tab's own memory ceiling: the file has to fit in RAM as a string plus its parsed form, so budget roughly three times the file size.
What each viewer is for
The four file types look similar and behave nothing alike.
JSON viewer — one value, arbitrarily deep. Use the tree, key and value search, and JSONPath filter to find things; use expand-to-level rather than expand-all on anything large.
JSONL / NDJSON viewer — many independent values, one per line. A malformed line is reported with its line number instead of failing the whole file, and the field profile shows which keys are actually present across records.
GeoJSON viewer — coordinates plus properties. It draws the geometry with no tile server involved and flags the two classic faults: a legacy crs member, and latitude and longitude the wrong way round.
HAR viewer — a browser network log. It reconstructs the waterfall, breaks each request into its DNS, connect, TLS, TTFB, and download phases, and masks credential headers on load.
Reading a file locally is the whole point
A HAR file contains every cookie and Authorization header the browser sent. A production JSON dump contains customer records. Uploading either to a stranger's server just to look at it is the thing security teams tell you not to do, and it is the reason these tools exist as browser pages rather than as an API. There is no upload endpoint on this site, and no payload is ever placed in a URL.