Why my `PureComponent` is re-rendered?

my-component.jsx

class MyComponent extends React.Component<IProps> {

  shouldComponentUpdate(nextProps, nextState) {
    const nextPropsKeys = Object.keys(nextProps)
    const propsKeys = Object.keys(this.props)

    if (nextPropsKeys.length !== propsKeys.length) {
      console.warn(`[MyComponent]: Mismatch in prop's keys`)
      return true;
    }

    for (let i = 0; i < nextPropsKeys.length; i++) {
      const key = nextPropsKeys[i];

      if (nextProps[key] !== this.props[key]) {
        console.warn(`[MyComponent]: nextProps.${key} is not equal to the current value. This causes re-render`)
        return true;
      }
    }


    const nextStateKeys = Object.keys(nextState)
    const stateKeys = Object.keys(this.state)

    if (nextStateKeys.length !== stateKeys.length) {
      console.warn(`[MyComponent]: Mismatch in state's keys`)
      return true;
    }

    for (let i = 0; i < nextStateKeys.length; i++) {
      const key = nextStateKeys[i];

      if (nextState[key] !== this.state[key]) {
        console.warn(`[MyComponent]: nextState.${key} is not equal to the current value. This causes re-render`)
        return true;
      }
    }

    return false;
  }

Motivation

This snippet allows you to precisely define what is causing a re-render of your PureComponent.

Real live example. You have a list of "List Item"'s. You call a render on the list. You are confident that only one specific "List Item" should be re-rendered. But for some reason, you see that other components are re-rendered(easy to see those re-renders using react-devtools). Okay, but why other items are updated?

How to use?

1. Change the "List Item" component extending from React.PureComponent but from React.Component;
2. Add a method shouldComponentUpdate from a snipper to "List Item";
3. Do your test scenario that causing re-render;
4. Check out the log's warnings. You should see what props forcing a not unexpected render.
5. Fix the problematic props and enjoy responsive UI

The example of log's warnings: