title: React state don't update
body: <p>I'm updating the state of an object, say like
  <code>obj = { a:1, b: ['hello','bye'] } </code> to
  <code>setObj({ a:1, b:['hi','good'] })</code>. The react re-renders this ParentComponent.</p>
  <p>//ParentComponent</p>
  <pre><code>return (
    &lt;&gt;
      {
        obj.b.map((data,i)=&gt;{
          &lt;ChildComponent greetings={data}/&gt;
        })
      }
    &lt;/&gt;
  )
  </code></pre>
  <p>But it doesn't re-render the ChildComponent, I'm new to react and still exploring new things. Please help me out with this one.</p>