JSON Formatter

What it does

A formatter does one thing: it takes JSON text and gives the same data back with the structure visible. The interesting part is not the indentation, it is what happens when the input is not JSON at all — a formatter that reformats it anyway has told you nothing.

Parse the input, re-serialise with the chosen indent, and return the text. If parsing fails, return the parser error with its position instead of output.

Why it works this way

Formatting cannot change what the data is — only how it is laid out. That is why a formatter is a safe place to paste something: if it comes back, it parsed.

A formatter that returns output for input that is not valid JSON is worse than one that errors, because the output looks like an answer.

Sorting keys changes the order of an object's members, which JSON does not consider significant. It makes two documents diffable by eye; it does not make them equal.

Examples

{"a":1,"b":[2,3]}

{
  "a": 1,
  "b": [
    2,
    3
  ]
}

{"a":1,}

error at line 1 column 8: unexpected }

Inputs and outputs

• JSON text
• indent width (2, 4, or tab)
• sort keys: on/off

• the formatted JSON text
• or the parse error, with line and column

Edge cases

• already indented — output is stable, not doubled
• empty string — error, not empty output
• very large input — indentation is not the bottleneck, parsing is
• BOM and leading whitespace

Sources

• RFC 8259, The JavaScript Object Notation (JSON) Data Interchange Format