`StrictMode` is a tool for highlighting potential problems in an application. Like `Fragment`, `StrictMode` does not render any visible UI. It 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:
`embed:strict-mode/enabling-strict-mode.js`
In the above example, strict mode checks will *not* be run against the `Header` and `Footer` components. However, `ComponentOne` and `ComponentTwo`, as well as all of their descendants, will have the checks.
`StrictMode` currently helps with:
* [Identifying components with unsafe lifecycles](#identifying-unsafe-lifecycles)
* [Warning about legacy string ref API usage](#warning-about-legacy-string-ref-api-usage)
As explained [in this blog post](/blog/2018/03/27/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:
Addressing the issues identified by strict mode _now_ will make it easier for you to take advantage of concurrent rendering in future releases of React.
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 recommendation was to [use the callback form instead](/docs/refs-and-the-dom.html#legacy-api-string-refs).
React used to support `findDOMNode` to search the tree for a DOM node given a class instance. Normally you don't need this because you can [attach a ref directly to a DOM node](/docs/refs-and-the-dom.html#creating-refs).
`findDOMNode` can also be used on class components but this was breaking abstraction levels by allowing a parent to demand that certain children were rendered. It creates a refactoring hazard where you can't change the implementation details of a component because a parent might be reaching into its DOM node. `findDOMNode` only returns the first child, but with the use of Fragments, it is possible for a component to render multiple DOM nodes. `findDOMNode` is a one time read API. It only gave you an answer when you asked for it. If a child component renders a different node, there is no way to handle this change. Therefore `findDOMNode` only worked if components always return a single DOM node that never changes.
You can instead make this explicit by passing a ref to your custom component and pass that along to the DOM using [ref forwarding](/docs/forwarding-refs.html#forwarding-refs-to-dom-components).
> In CSS, the [`display: contents`](https://developer.mozilla.org/en-US/docs/Web/CSS/display#display_contents) attribute can be used if you don't want the node to be part of the layout.
* The **render** phase determines what changes need to be made to e.g. the DOM. During this phase, React calls `render` and then compares the result to the previous render.
* The **commit** phase is when React applies any changes. (In the case of React DOM, this is when React inserts, updates, and removes DOM nodes.) React also calls lifecycles like `componentDidMount` and `componentDidUpdate` during this phase.
The commit phase is usually very fast, but rendering can be slow. For this reason, the upcoming concurrent mode (which is not enabled by default yet) breaks the rendering work into 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).
*`setState` updater functions (the first argument)
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 can often be [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 functions:
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.
> In React 17, React automatically modifies the console methods like `console.log()` to silence the logs in the second call to lifecycle functions. However, it may cause undesired behavior in certain cases where [a workaround can be used](https://github.com/facebook/react/issues/20090#issuecomment-715927125).
>
> Starting from React 18, React does not suppress any logs. However, if you have React DevTools installed, the logs from the second call will appear slightly dimmed. React DevTools also offers a setting (off by default) to suppress them completely.
The legacy context API is error-prone, and will be removed in a future major version. It still works for all 16.x releases but will show this warning message in strict mode:
In the future, we’d like to add a feature that allows React to add and remove sections of the UI while preserving state. For example, when a user tabs away from a screen and back, React should be able to immediately show the previous screen. To do this, React will support remounting trees using the same component state used before unmounting.
This feature will give React better performance out-of-the-box, but requires components to be resilient to effects being mounted and destroyed multiple times. Most effects will work without any changes, but some effects do not properly clean up subscriptions in the destroy callback, or implicitly assume they are only mounted or destroyed once.
To help surface these issues, React 18 introduces a new development-only check to Strict Mode. This new check will automatically unmount and remount every component, whenever a component mounts for the first time, restoring the previous state on the second mount.
To demonstrate the development behavior you'll see in Strict Mode with this feature, consider what happens when React mounts a new component. Without this change, when a component mounts, React creates the effects:
```
* React mounts the component.
* Layout effects are created.
* Effects are created.
```
With Strict Mode starting in React 18, whenever a component mounts in development, React will simulate immediately unmounting and remounting the component:
```
* React mounts the component.
* Layout effects are created.
* Effect effects are created.
* React simulates effects being destroyed on a mounted component.
* Layout effects are destroyed.
* Effects are destroyed.
* React simulates effects being re-created on a mounted component.
* Layout effects are created
* Effect setup code runs
```
On the second mount, React will restore the state from the first mount. This feature simulates user behavior such as a user tabbing away from a screen and back, ensuring that code will properly handle state restoration.
When the component unmounts, effects are destroyed as normal:
