JavaScript Transform

Source: 05_javascript_transform

An HTTP server that applies a JavaScript function to every incoming JSON payload and returns the transformed result. The JS engine is declared as a model so the function is compiled once at startup and reused across all concurrent requests.

Running

melodium run 05_javascript_transform/Compo.toml

$ curl -X POST http://127.0.0.1:8080/transform \
       -H "Content-Type: application/json" \
       -d '{"name":"Alice","score":82}'
{"name":"Alice","score":82,"grade":"B","note":"processed by Mélodium JS engine"}

How it works

The Transformer model embeds the JavaScript function directly in the source:

model Transformer() : JavaScriptEngine {
    code = "
function transform(input) {
    var grade = 'F';
    if (input.score >= 90) grade = 'A';
    …
    return { name: input.name, score: input.score, grade: grade, … };
}
"
}

The JS engine compiles this function once when the model is instantiated. Every request then calls process[engine=engine](code="transform(value)") which invokes the compiled function with the parsed JSON value.

The jsTransform sub-treatment

Self.data -> decode.data,text -> toJson.text,json -> unwrap.option,value
          -> process.value,result -> unwrapOr.option,value
          -> toString.value,into -> encode.text,data -> Self.data

Two different unwrap strategies are used deliberately:

• unwrap<Json>() before process — malformed JSON is a hard error; there is nothing useful to pass to the JS function
• unwrapOr<Json>(default=|null()) after process — a JS runtime error produces a graceful null response rather than crashing the request

The jsTransform sub-treatment declares its own model engine: Transformer(), which means the model instance is shared with main’s engine through Mélodium’s model propagation — the function is not recompiled per request.

Dependencies

[dependencies]
std        = "0.10.1"  # core flows, logging, data structures
http       = "0.10.1"  # HTTP server and client
net        = "0.10.1"  # IP address helpers
json       = "0.10.1"  # JSON parsing and serialisation
encoding   = "0.10.1"  # UTF-8 encode / decode
javascript = "0.10.1"  # embedded JavaScript engine