node-red/packages/node_modules/@node-red/nodes/locales/en-US/function/10-function.html

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<script type="text/html" data-help-name="function">
    <p>A JavaScript function to run against the messages being received by the node.</p>
    <p>The messages are passed in as a JavaScript object called <code>msg</code>.</p>
    <p>By convention it will have a <code>msg.payload</code> property containing
       the body of the message.</p>
    <p>The function is expected to return a message object (or multiple message objects), but can choose
       to return nothing in order to halt a flow.</p>
    <p>The <b>Setup</b> tab contains code that will be run whenever the node is started.
        The <b>Close</b> tab contains code that will be run when the node is stopped.</p>
    <p>If an promise object is returned from the setup code, input message processing starts after its completion.</p>
    <h3>Details</h3>
    <p>See the <a target="_blank" href="http://nodered.org/docs/writing-functions.html">online documentation</a>
    for more information on writing functions.</p>
    <h4>Sending messages</h4>
    <p>The function can either return the messages it wants to pass on to the next nodes
    in the flow, or can call <code>node.send(messages)</code>.</p>
    <p>It can return/send:</p>
    <ul>
      <li>a single message object - passed to nodes connected to the first output</li>
      <li>an array of message objects - passed to nodes connected to the corresponding outputs</li>
    </ul>
    <p>Note: The setup code is executed during the initialization of nodes. Therefore, if <code>node.send</code> is called in the setup tab, subsequent nodes may not be able to receive the message.</p>
    <p>If any element of the array is itself an array of messages, multiple
          messages are sent to the corresponding output.</p>
    <p>If null is returned, either by itself or as an element of the array, no
          message is passed on.</p>
    <h4>Logging and Error Handling</h4>
    <p>To log any information, or report an error, the following functions are available:</p>
      <ul>
          <li><code>node.log("Log message")</code></li>
          <li><code>node.warn("Warning")</code></li>
          <li><code>node.error("Error")</code></li>
      </ul>
    </p>
    <p>The Catch node can also be used to handle errors. To invoke a Catch node,
    pass <code>msg</code> as a second argument to <code>node.error</code>:</p>
    <pre>node.error("Error",msg);</pre>
    <h4>Accessing Node Information</h4>
    <p>In the function block, id and name of the node can be referenced using the following properties:</p>
    <ul>
        <li><code>node.id</code> - id of the node</li>
        <li><code>node.name</code> - name of the node</li>
    </ul>
    <h4>Using environment variables</h4>
    <p>Environment variables can be accessed using <code>env.get("MY_ENV_VAR")</code>.</p>
</script>