How to Format, Validate and Minify JSON

The three things a formatter does

Validation runs a parser over the document and either returns the parsed value or a precise error location. A good validator tells you the line and character where parsing failed, which is half the battle.

Pretty-printing adds indentation and puts one key-value pair on each line. The data does not change - only whitespace does - so a pretty-printed document carries exactly the same information in a shape your eyes can follow.

Minifying removes all insignificant whitespace, collapsing the document to one line. That is what you usually want to send over a network or store in a build artifact.

{"name":"SwiftTools","tools":50,"free":true}

{
  "name": "SwiftTools",
  "tools": 50,
  "free": true
}

Common JSON syntax errors

Nearly every "my JSON is broken" case is one of these. They all parse fine in JavaScript, which is how they sneak in:

• Trailing comma - a comma after the last item in an array or object. JSON does not allow it, even though many languages do.
• Unquoted keys - { name: "x" } is invalid; keys must be double-quoted strings.
• Single quotes - JSON only accepts double quotes around string values.
• Comments - neither // nor /* */ is part of the format, and they always fail validation.
• NaN, Infinity and undefined - none of these exist in JSON; use null or omit the field.
• Duplicate keys - technically parseable, but most tools keep only the last value, which silently hides data.

A workflow for debugging API responses

When something "just does not work", keep this sequence in mind:

1. 1Capture the raw response - from your client code, a REST client, or the browser network tab. Save it exactly as received; do not edit it yet.
2. 2Validate it. If it parses, your problem is not the syntax but the shape or values - jump straight to reading the formatted output.
3. 3Pretty-print it. With indentation, structure becomes visible: you can see at a glance which nesting level is not what you expected.
4. 4Search the formatted document for the field your code expects. Mismatched key names (snake_case vs camelCase) are a classic cause of "null" values.
5. 5Minify it if you need to paste it into a bug report - one line is far easier to share than 300.

Formatting versus parsing

A document can be valid JSON and still not do what you expect. Formatting only makes structure visible; it cannot tell you that a field has the wrong type or that a value is off by a factor of ten. When the document validates but behaves wrongly, switch from format tools to data tools: check the types of the fields you rely on, and compare the payload against the documented schema.

The adjacent problems in most developer workflows are encoding and identity. URL encoding handles query parameters that contain JSON; Base64 appears in auth headers and data URIs; UUIDs are usually how the objects you are formatting are named in the first place. The URL Encoder, Base64 tool and UUID generator cover all three without leaving the browser.

Frequently asked questions