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

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    <p>A JavaScript function block 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>
    <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>
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          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>
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