node-red/nodes/core/core/80-function.html

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<script type="text/x-red" data-help-name="function">
    <p>A function block where you can write code to do more interesting things.</p>
    <p>The message is 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>
    <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")</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>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>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>
    <p>See the <a target="_new" href="http://nodered.org/docs/writing-functions.html">online documentation</a> for more help.</p>

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