Helios API/

Generating datums and redeemers

Smart contract transactions include datum and redeemer data. You can generate these data structures using Helios.

Let's look at the following Helios script (as a literal string inside a js file):

const src = `
spending owner_only

struct Datum {
    owner: PubKeyHash

func main(datum: Datum, _, ctx: ScriptContext) -> Bool {

const MY_DATUM = Datum {

Remember that after the main function you can define data generators and test functions (see script structure).

MY_DATUM in this example can be evaluated using the API:

// at top of js file
import * as helios from "helios"
const program = helios.Program.new(src)

const myDatum = program.parameters["MY_DATUM"]

Here myDatum is a UplcValue instance. UplcValue is the internal (unexported) base class of every Helios value. To get the underlying data we can use the data getter:

const myDatumData = myDatum.data

Note: the UplcValue data getter doesn't work for booleans as booleans are always kept in their primitive Plutus-Core form for performance reasons.

Here myDatumData is a UplcData instance. UplcData is equivalent to the BuiltinData type in Plutus.

To create a JSON string that can be used by cardano-cli we can use the toSchemaJson method:


// prints '{"constructor": 0, "fields": [...]}'