Write a value to an OPC UA server in Node.js

The code

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

// umati is read-only. Start a local server first:
//   npm i node-opcua-samples && npx simple_server
const URL = "opc.tcp://localhost:26543";

(async () => {
  const client = OPCUAClient.create({ endpointMustExist: false });
  await client.connect(URL);
  const session = await client.createSession();

  const nodeId = "ns=1;s=MyVariable";

  // read the DataType attribute first - do not guess
  const dt = await session.read({ nodeId, attributeId: AttributeIds.DataType });
  console.log("declared dataType:", dt.value.value.toString());

  const status = await session.write({
    nodeId,
    attributeId: AttributeIds.Value,
    value: { value: { dataType: DataType.Double, value: 42.5 } },
  });
  console.log("write status     :", status.toString());

  const back = await session.read({ nodeId, attributeId: AttributeIds.Value });
  console.log("read back        :", back.value.value);

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

What it prints

declared dataType: ns=0;i=11
write status     : Good (0x00000000)
read back        : 42.5

Notes

• Read the DataType attribute and match it. DataType.Double is not DataType.Float, and the server will tell you so.
• A Good write status does not prove the value changed. Read back in tests: some servers accept and discard, deliberately.
• Check AccessLevel before writing rather than interpreting the failure afterwards.
• For arrays, supply the full array or an arrayType and IndexRange. A scalar into an array node is a type mismatch.

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.