Decode custom structures in Node.js

The code

const { OPCUAClient, AttributeIds } = require("node-opcua");

(async () => {
  const client = OPCUAClient.create({ endpointMustExist: false });
  await client.connect("opc.tcp://opcua.umati.app:4840");
  const session = await client.createSession();

  // populate the session's data type manager from the server
  await session.extractNamespaceDataType();

  const dv = await session.read({
    nodeId: "ns=4;i=7001",
    attributeId: AttributeIds.Value,
  });

  const v = dv.value.value;
  console.log("decoded as:", v.constructor.name);
  console.log(JSON.stringify(v, null, 2));

  await session.close();
  await client.disconnect();
})();

What it prints

decoded as: ProductionJobType
{
  "identifier": "JOB-2026-0912",
  "state": 2,
  "partsPlanned": 500,
  "partsProduced": 431
}

Notes

• Call extractNamespaceDataType() after creating the session and before reading structures, or you get an opaque ExtensionObject.
• If decoding still fails, the server may publish only a legacy binary type dictionary rather than DataTypeDefinition attributes. That is an interoperability gap between the two mechanisms.
• Unions and optional fields are the usual breaking point. Test them explicitly.
• Field names arrive camelCased in JavaScript even though the model declares them PascalCased.

What breaks first

Last verified 7 September 2026 against OPC UA 1.05.07. OPC UA Connect is an educational resource and is not affiliated with the OPC Foundation.