`StrictMode` is a tool for highlighting potential problems in an application. Like `Fragment`, `StrictMode` does not render any visible UI. It simply activates additional checks and warnings for its descendants.
> Note:
>
> Strict mode checks are run in development mode only; _they do not impact the production build_.
You can enable strict mode for any part of your application. For example:
In the above example, strict mode checks will *not* be run against the `Header` and `Footer` components. However, `RouteOne` and `RouteTwo`, as well as all of their descendants, will have the checks.
As explained [in this blog post](/blog/2018/02/07/update-on-async-rendering.html), certain legacy lifecycle methods are unsafe for use in async React applications. However, if your application uses third party libraries, it can be difficult to ensure that these lifecycles aren't being used. Fortunately, strict mode can help with this!
When strict mode is enabled, React compiles a list of all class components using the unsafe lifecycles, and logs a warning message with information about these components, like so:
Previously, React provided two ways for managing refs: the legacy string ref API and the callback API. Although the string ref API was the more convenient of the two, it had [several downsides](https://github.com/facebook/react/issues/1373) and so our official recomendation was to [use the callback form instead](https://reactjs.org/docs/refs-and-the-dom.html#legacy-api-string-refs).
Version 16.3 adds a new option for managing refs that offers the convenience of a string ref without any of the downsides:
`embed:16-3-release-blog-create-ref.js`
> **Note:**
>
> Callback refs will continue to be supported in addition to the new `createRef` API.
>
> You don't need to replace callback refs in your components. They are slightly more flexible, so they will remain as an advanced feature.
* The **render** phase determines what changes need to be made to e.g. the DOM. During this phase, React calls lifecycles like `getDerivedStateFromProps` and `render` and then compares the result to the previous render.
* The **commit** phase is when React applies any changes. (In the case of ReactDOM, this is when React inserts, updates, and removes DOM nodes.) React calls lifecycles like `componentDidMount` and `componentDidUpdate` during this phase.
The commit phase is very fast, but rendering can be slow. For this reason, async mode breaks rendering into multiple pieces, pausing and resuming the work to avoid blocking the browser. This means that React may invoke render phase lifecycles more than once before committing, or it may invoke them without committing at all (because of an error or a higher priority interruption).
Render phase lifecycles include the following class component methods:
*`constructor`
*`componentWillMount`
*`componentWillReceiveProps`
*`componentWillUpdate`
*`getDerivedStateFromProps`
*`shouldComponentUpdate`
*`render`
*`setState` updater functions
Because the above methods might be called more than once, it's important that they do not contain side-effects. Ignoring this rule can lead to a variety of problems, including memory leaks and invalid application state. Unfortunately, it can be difficult to detect these problems as they are often [non-deterministic](https://en.wikipedia.org/wiki/Deterministic_algorithm).
Strict mode can't automatically detect side effects for you, but it can help you spot them by making them a little more deterministic. This is done by intentionally double-invoking the following methods:
* Class component `constructor` method
* The `render` method
*`setState` updater functions
* The static `getDerivedStateFromProps` lifecycle
> Note:
>
> This only applies to development mode. _Lifecycles will not be double-invoked in production mode._
At first glance, this code might not seem problematic. But if `SharedApplicationState.recordEvent` is not [idempotent](https://en.wikipedia.org/wiki/Idempotence#Computer_science_meaning), then instantiating this component multiple times could lead to invalid application state. This sort of subtle bug might not manifest during development, or it might do so inconsistently and so be overlooked.
