Why does this tool take a hex string instead of a binary file?

Browsers work with text, not raw binary, so pasting a hex dump (two characters per byte) is the practical way to hand BSON bytes to a browser-based tool. It accepts exactly the same hex format the JSON to BSON Converter on this site outputs.

Which BSON element types can this decoder read?

Double, string, embedded document, array, boolean, null, int32, and int64, the same subset that JSON to BSON Converter produces. That covers everything a JSON document round-tripped through BSON would contain.

What happens with MongoDB-specific types like ObjectId or Date?

This decoder targets the BSON subset that corresponds directly to JSON's data model; MongoDB-specific extended types like ObjectId, Date, or Binary aren't part of what it decodes since plain JSON has no equivalent representation for them.

What happens if the hex string is malformed or truncated?

The decoder reports an error describing what it expected and where, since a malformed length prefix or a truncated element mid-document means the rest of the bytes can't be reliably interpreted as BSON.

BSON to JSON Converter - Decode BSON Online

Overview

Introduction

When you're debugging a MongoDB wire-protocol capture, a driver's encoder, or a BSON fixture, you often end up staring at a hex dump of bytes with no easy way to see what document it actually represents.

This tool decodes that hex-encoded BSON back into readable JSON directly in the browser, no MongoDB driver or server required.

What Is BSON to JSON Converter?

A BSON decoder that reads a hex-encoded byte sequence, its length-prefixed document structure, typed elements, and null terminator, and reconstructs the equivalent JSON value.

It decodes the same element types the site's JSON to BSON Converter produces: doubles, strings, embedded documents, arrays, booleans, null, int32, and int64.

How BSON to JSON Converter Works

The decoder first turns the hex text into raw bytes, then reads the leading 4-byte little-endian length prefix and walks each typed element in sequence, using each element's type byte to know how many following bytes to consume and how to interpret them.

Embedded documents and arrays are decoded by recursively applying the same document-reading logic, with array elements distinguished by their numeric string keys ("0", "1", ...) the way the BSON spec defines arrays.

When To Use BSON to JSON Converter

Use it when you have a BSON hex dump from a wire-protocol capture, driver test, or the JSON to BSON Converter and need to see what document it actually decodes to.

It's also useful for learning BSON's binary layout by pairing a hex dump with its decoded JSON side by side.

Often used alongside JSON to BSON Converter, JSON Formatter & Beautifier and JSON Validator.

Features

Advantages

Decodes directly in the browser with no MongoDB driver or server needed.
Reports a specific error when the byte sequence is malformed rather than failing silently.
Accepts the exact hex format the companion JSON to BSON Converter produces, so the pair round-trips cleanly.

Limitations

Doesn't decode MongoDB extended types like ObjectId, Date, or Binary, since those have no direct JSON equivalent.
Expects a hex-encoded string, not a raw uploaded .bson binary file.

Examples

Decoding a small document

Input

2e 00 00 00 10 5f 69 64 00 01 00 00 00 02 6e 61 6d 65 00 0d 00 00 00 41 64 61 20 4c 6f 76 65 6c 61 63 65 00 08 61 63 74 69 76 65 00 01 00

Output

{
  "_id": 1,
  "name": "Ada Lovelace",
  "active": true
}

The int32 _id, string name, and boolean active elements are read in order and reassembled into a JSON object.

Invalid BSON

Input

2e 00 00 00 10 5f 69

Output

Invalid BSON: unexpected end of input while reading element

A hex string that's truncated mid-element is rejected with a description of what went wrong.

Best Practices & Notes

Best Practices

Use the exact hex output from JSON to BSON Converter when testing the pair, rather than hand-editing bytes, to avoid introducing an invalid length prefix.
Strip any non-hex formatting (like a 0x prefix per byte) before pasting, the decoder expects plain space-separated hex pairs.
Cross-check unexpected output against the BSON spec's type byte table when a document decodes but looks wrong.

Developer Notes

The decoder reads bytes with a DataView-backed cursor that advances by exactly the number of bytes each type consumes (4 for int32, 8 for double/int64, a length-prefixed run for strings), rather than regex-based parsing, since BSON's binary layout is fundamentally positional and has no textual delimiters to scan for.

BSON to JSON Converter Use Cases

Inspecting a BSON hex dump captured from MongoDB's wire protocol
Verifying that a driver or custom encoder produced spec-correct BSON bytes
Round-tripping a document through JSON to BSON Converter and back to confirm no data was lost

Common Mistakes

Pasting a raw binary .bson file's contents instead of its hex representation.
Expecting ObjectId or Date values to decode to something other than their raw byte representation, since those extended types aren't part of this decoder's scope.
Including formatting like 0x prefixes or commas between hex pairs, which will break parsing.

Tips

Pair with JSON to BSON Converter to round-trip a document and confirm both directions agree.
If a document fails to decode, check the length prefix first, an incorrect total length is the most common cause of a truncated read.

BSON to JSON Converter