title: node-red parsing a json string to msg.payload
body: <p>Noob question</p>
  
  <p>I am using the snmp function to collect data via oid lists.
  I have extracted the data and used the json function to parse the data into a json string seen below.</p>
  
  <blockquote>
    <p>msg.payload : string[81]
    <code>"[{"oid":"1.3.6.1.4.1.38783.3.3.1.1.1.0","type":2,"value":53800,"tstr":"Integer"}]"</code></p>
  </blockquote>
  
  <p>I am trying to write a function to strip out <code>"value":53800</code> from this string and output it in <code>msg.payload</code>.</p>
  
  <p>I have tried below but it returns </p>
  
  <blockquote>
    <p>"TypeError: Cannot assign to read only property <code>'_msgid'</code> of <code>"value":53700,"tstr":"Integer"}]"</code></p>
  </blockquote>
  
  <pre><code>var msg = msg.payload;
  var value = msg.substr(49,62);
  
  return value;
  </code></pre>