title: Flow type, What does the `+` symbol mean in front a property?
body: <p>I came across the following code written in js FlowType (I am interested to know the value of + in the context of FlowType not in general JS).</p>
  
  <p>Could you please explain me what does the <code>+</code> symbol mean in front of the property in the code below:</p>
  
  <pre><code>  export type User = {
        +name: string,
        +surname: string,
        +personId: PourceId,
      }
  </code></pre>
  
  <p>I could not find any reference in the documentation, any link is also welcome. </p>