title: Modify data using AJV for Json Schema
body: <p>I'm using AJV (Another Json schema validator) on NodeJs. </p>
  
  <p>I've the following schema</p>
  
  <pre><code>var schema = {
      "$id": "testSchema.json",
      "type": "object",
      "$schema": "http://json-schema.org/draft-06/schema#",
      "additionalProperties": false,
      "properties": {
          "userId": {
              "type": "integer"
          },
          "userName": {
              "type": "string"
          },
          "uniqueID": {
              "type": "integer"
          }
      }
  }
  </code></pre>
  
  <p>I need to overwrite <code>unqiueID</code> property by a value that I could somehow pass to Json schema or AJV.
  I think the above can be done using AJV addKeyword method, tried using it but failed because I don't know how to manipulate (and return) data value from AJV custom keywords. 
  Is possible to modify data with AJV ? or are there any other possible ways to do it??
  Thank you!</p>