# Format JSON without losing numeric precision

Preserve number and escape tokens, export complete arrays, and understand which whitespace the JSON formatter changes.

Workspace 0.2.2 · Analyzer 0.1.0, formatter 0.1.0, checker 0.1.0 · Updated September 14, 2026

[Read this guide as HTML](https://hl7tools.io/guides/lossless-json.html) · [Open JSON formatter](https://hl7tools.io/fhir-response-formatter.html)

## Preserve what the sender wrote

The [FHIR Response Formatter](https://hl7tools.io/fhir-response-formatter.html) accepts strict JSON, including objects, arrays and scalar values. It preserves number spelling, string and key escapes, member order and array order while changing formatting whitespace.

The [FHIR R4 JSON guidance](https://hl7.org/fhir/R4/json.html#primitive) explains that native JavaScript number parsing can lose decimal precision and trailing zeros. Here, `2.00` remains `2.00`, and a large integer does not pass through a JavaScript Number conversion.

## Try the token-preservation example

Paste this synthetic JSON array. The resource-shaped objects demonstrate formatting only; the formatter does not check whether they are valid FHIR resources.

```json
[
 {"resourceType":"Observation","valueQuantity":{"value":2.00},"large":9007199254740993,"label":"A\u0042"},
 {"resourceType":"Patient","id":"synthetic-only","note":"line\nnext"}
]
```

Expected properties of the formatted output:

```text
Numeric tokens: 2.00; 9007199254740993
String tokens: "A\u0042"; "line\nnext"
Resource inventory: Observation, Patient
Exported top-level items: 2
```

The escaped label can decode to `AB` for inspection, while its exported token remains `"A\u0042"`. The resource menu identifies objects with a string `resourceType`; it makes no validity claim.

## Export the complete document

Selecting a resource changes the preview. Copy and Download still use the entire formatted document, including both array items above. Download requests a browser download; the site cannot confirm that you saved the file.

In module code, export `result.formatted` or `result.compact` directly. The inspection tree stores numbers as `JSONNumber` objects with a `raw` token. Do not reconstruct output with `Number`, `JSON.stringify(result.value)` or text read from the preview; numeric wrappers reject conversion and serialization.

## Resolve ambiguous keys first

This input is rejected with the error code below. The two key tokens decode to the same name, so retaining only one would silently discard a value.

```json
{"id":"first","\u0069d":"second"}
```

```text
DUPLICATE_KEY
```

## Understand the word lossless

- Whitespace inside strings remains intact. Indentation, line breaks and other insignificant whitespace outside strings may change; this is not an original-byte archive.
- Neither formatted nor compact output is a signature canonicalization format. Formatting also does not establish FHIR conformance.
- Comments, trailing commas, duplicate keys and JavaScript expressions are rejected.
- Limits are 1 MiB of UTF-8 input, 50,000 JSON values, 128 nested containers and 8,388,608 output characters. A resource menu can show at most 2,000 resources; complete output remains available.

All examples are synthetic. [Browse all developer guides](https://hl7tools.io/guides.html).