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That is because setState() calls the render() function of your component.

Last year, I created this diagram

You can see that after therender() function, the componentDidMount() function will be called by React. When you put a setState() call in componentDidMount() then you are causing the entire component tree be re-rendered not only the current component - not to forget, the current component did just finished with rendering.

What you really want to use is props instead of state.

Edit: The picture ...

Initial Render - - - - - - - - -> Props Change - - - - - - - - - -> State Change - - - - - - - - - -> Component Unmount

+---------------------------+                                                                                                      
| displayName               |                                                                                                      
+---------------------------+                                                                                                      

+---------------------------+                                                                                                      
| statics                   |                                                                                                      
+---------------------------+                                                                                                      

+---------------------------+                                                                                                      
| mixins                    |                                                                                                      
+-------------+-------------+                                                                                                      
              |                                                                                                                    
+-------------v-------------+                                                                                                      
| propTypes                 |                                                                                                      
+-------------+-------------+                                                                                                      
              |                                                                                                                    
+-------------v-------------+     +---------------------------+                                                                    
| getInitialProps           |     | componentWillReceiveProps |                                                                    
+-------------+-------------+     +-------------+-------------+                                                                    
              |                                 |                                                                                  
+-------------v-------------+     +-------------v-------------+      +---------------------------+                                 
| getInitialState           |     | shouldComponentUpdate     |      | shouldComponentUpdate     |                                 
+-------------+-------------+     +-------------+-------------+      +-------------+-------------+                                 
              |                                 |                                  |                                               
+-------------v-------------+     +-------------v-------------+      +-------------v-------------+                                 
| componentWillMount        |     | componentWillUpdate       |      | componentWillUpdate       |                                 
+-------------+-------------+     +-------------+-------------+      +-------------+-------------+                                 
              |                                 |                                  |                                               
+-------------v-------------+     +-------------v-------------+      +-------------v-------------+                                 
| render                    |     | render                    |      | render                    |                                 
+-------------+-------------+     +-------------+-------------+      +-------------+-------------+                                 
              |                                 |                                  |                                               
+-------------v-------------+     +-------------v-------------+      +-------------v-------------+                                 
| componentDidMount         |     | componentDidUpdate        |      | componentDidUpdate        |                                 
+-------------+-------------+     +-------------+-------------+      +-------------+-------------+                                 
              |                                 |                                  |                                               
              |                                 |                                  |                  +---------------------------+
              +---------------------------------+----------------------------------+------------------> componentWillUnmount      |
                                                                                                      +---------------------------+