title: typescript generic type with equal operator means?
body: <p>I am learning typescript generic, and come across the following generic type with the equal operator for extends type</p>
  
  <pre><code>export interface DataType {
    [key: string]: FieldValue;
  }
  
  export interface FormProps&lt;Data extends DataType = DataType&gt; { }
  </code></pre>
  
  <p>What does <code>DataType = DataType</code> mean in here?</p>