Bracket Notation

Bracket notation encodes structured data in flat key-value formats such as query strings and form data. OpenAPI Serializer, OpenAPI Handler, and OpenAPI Link use it whenever nested data must be represented outside plain JSON.

Rules

1. Repeated keys become arrays.

   color=red&color=blue -> { color: ['red', 'blue'] }

2. Append [] to push into an array.

   color[]=red&color[]=blue -> { color: ['red', 'blue'] }

3. Append [number] to target an explicit array index.

   color[0]=red&color[2]=blue -> { color: ['red', <empty>, 'blue'] }

   INFO

   Missing indexes create sparse arrays.

   Explicit indexes greater than 999 are treated as object keys by default to avoid huge sparse arrays during deserialization. To change that limit, configure maxExplicitDeserializingArrayIndex:

   const serializer = new OpenAPISerializer({
     bracketNotation: {
       maxExplicitDeserializingArrayIndex: 1999,
     }
   })

4. Append [key] to target an object property.

   color[red]=true&color[blue]=false -> { color: { red: 'true', blue: 'false' } }

Limitations

Bracket notation is designed to express structured data in constrained environments, so it has a few unavoidable limitations:

• Cannot represent empty structures like empty objects {} or empty arrays [].
• Cannot represent an array at the root level. For example, 0=red&1=blue becomes { 0: 'red', 1: 'blue' }, not ['red', 'blue'].
• Cannot represent objects whose keys are all numbers, because they can be mistaken for array indexes.

INFO

If bracket notation is used in query strings or form data, it also inherits the limitations of those formats. For example, values are always strings or files, and null or undefined cannot be represented.

Examples

URL Query

curl 'http://example.com/api/example?name[first]=John&name[last]=Doe'

This query is parsed as:

{
  "name": {
    "first": "John",
    "last": "Doe"
  }
}

Form Data

curl -X POST http://example.com/api/example \
  -F 'name[first]=John' \
  -F 'name[last]=Doe'

This form data is parsed as:

{
  "name": {
    "first": "John",
    "last": "Doe"
  }
}

Complex Example

curl -X POST http://example.com/api/example \
  -F 'data[names][0][first]=John1' \
  -F 'data[names][0][last]=Doe1' \
  -F 'data[names][1][first]=John2' \
  -F 'data[names][1][last]=Doe2' \
  -F 'data[ages][0]=18' \
  -F 'data[ages][2]=25' \
  -F 'data[files][]=@/path/to/file1' \
  -F 'data[files][]=@/path/to/file2'

This form data is parsed as:

{
  "data": {
    "names": [
      { "first": "John1", "last": "Doe1" },
      { "first": "John2", "last": "Doe2" }
    ],
    "ages": ["18", "<empty>", "25"],
    "files": ["<binary data>", "<binary data>"]
  }
}

Learn More

The bracket notation is a small, self-contained module, making it easy to understand. To explore its behavior in detail, see the source code.