transform-javascript

JavaScript body transform

Last modified: 2026-04-27

Demonstrates the javascript transform via QuickJS. The entrypoint is function transform(body) where body is the raw upstream body as a string. The script parses it as JSON, mutates the document, and returns a new JSON string. A static action seeds the input so the example runs offline. The script computes title_length, adds a reversed copy of the title, trims body to 40 characters with an ellipsis, and stamps transformed_by = "javascript". The origin is reached on 127.0.0.1:8080 via the js.local Host header.

Run

sbproxy serve -f sb.yml

Try it

# Upstream body (what the static action emits internally):
# {
#   "id": 1,
#   "title": "javascript runs on the request path",
#   "body": "this body is longer than forty characters so it will be trimmed by the script",
#   "userId": 7
# }

# Client response after the javascript transform
$ curl -s -H 'Host: js.local' http://127.0.0.1:8080/ | jq
{
  "id": 1,
  "title": "javascript runs on the request path",
  "body": "this body is longer than forty character...",
  "userId": 7,
  "title_length": 35,
  "title_reversed": "htap tseuqer eht no snur tpircsavaj",
  "transformed_by": "javascript"
}

# title_length matches the source title length (no rename)
$ curl -s -H 'Host: js.local' http://127.0.0.1:8080/ | jq '.title_length'
35

# body is trimmed to 40 chars + "..."
$ curl -s -H 'Host: js.local' http://127.0.0.1:8080/ | jq -r '.body | length'
43

# Provenance tag is added
$ curl -s -H 'Host: js.local' http://127.0.0.1:8080/ | jq -r '.transformed_by'
javascript

What this exercises

• javascript transform via QuickJS with the canonical function transform(body) entrypoint
• String-shaped input: the script parses and re-serialises the JSON itself (JSON.parse / JSON.stringify)
• Field derivation, slicing, and provenance stamping
• static action - inline JSON body so the example runs offline

See also

• docs/features.md - full feature reference
• docs/configuration.md - configuration schema
• docs/scripting.md - Lua, JavaScript, and CEL scripting reference