react strict mode зачем нужен
3.18 Строгий режим
Проверки строгого режима работают только в режиме разработки; они не влияют на production билд.
Вы можете включить строгий режим для любой части вашего приложения. Например:
На данный момент StrictMode помогает с:
Дополнительные функциональные возможности будут добавлены в будущих релизах React.
3.18.1 Обнаружение компонентов, имеющих небезопасные методы жизненного цикла
Некоторые устаревшие методы жизненного цикла небезопасны для использования в асинхронных React-приложениях. Однако, если ваше приложение использует сторонние библиотеки, может оказаться сложным обеспечить, чтобы эти методы не использовались. К счастью, строгий режим может помочь с этим!
Когда строгий режим включен, React компилирует список всех компонентов-классов, использующих небезопасные методы жизненного цикла, и отображает предупреждающее сообщение с информацией об этих компонентах, например:
Теперь, решение проблем, выявленных в строгом режиме, облегчит использование вами всех преимуществ асинхронной отрисовки в будущих версиях React.
3.18.2 Предупреждение об использовании устаревшего строкового API для ref
Ранее React предоставлял два способа управления ссылками ref : устаревший строковый API и API обратного вызова. Хотя строковый API был более удобным, он имел ряд недостатков, поэтому наша официальная рекомендация заключалась в том, чтобы вместо него использовать форму обратного вызова.
React 16.3 добавил третий вариант, который предлагает удобство строки ref без каких-либо недостатков:
Вам не нужно заменять обратные вызовы в ваших компонентах. Они немного более гибкие, поэтому останутся в качестве продвинутой функции.
3.18.3 Предупреждением об использовании устаревшего метода findDOMNode
Ранее React использовал метод findDOMNode для поиска узла DOM по заданному экземпляру класса. Обычно вам это не нужно, так как вы можете прикрепить ссылку непосредственно к узлу DOM.
Вместо этого можно использовать передачу ссылок.
Вы также можете добавить обёртку дла DOM-узла в свой компонент и прикрепить ссылку непосредственно к ней.
3.18.3 Обнаружение неожиданных сторонних эффектов
Концептуально, React работает в две фазы:
К методам жизненного цикла фазы отрисовки относятся следующие методы компонента-класса:
Поскольку вышеупомянутые методы могут быть вызваны более одного раза, важно, чтобы они не содержали каких-либо сторонних эффектов. Игнорирование этого правила может привести к множеству проблем, включая утечку памяти и нерелевантное состояние приложения. К сожалению, бывает довольно трудно обнаружить эти проблемы, поскольку они часто могут быть недетерминированными.
Строгий режим не может автоматически обнаруживать для вас побочные эффекты, но он может помочь вам определить их, сделав их немного более детерминированными. Это достигается путем преднамеренного двойного вызова следующих методов:
Это применимо только к development режиму. Методы жизненного цикла не будут вызываться дважды в production режиме.
К примеру, рассмотрим следующий код:
На первый взгляд данный код может не показаться проблемным. Но если метод SharedApplicationState.recordEvent не является идемпотентным, то создание экземпляра этого компонента несколько раз может привести к недопустимому состоянию приложения. Такая тонкая ошибка может не проявляться во время разработки, или же она может возникать очень непоследовательно, и поэтому может быть упущена из виду.
Умышленно производя двойные вызовы методов, таких как конструктор компонента, строгий режим делает такие проблемные шаблоны более легкими для обнаружения.
3.18.5 Обнаружением устаревшего API контекста
Устаревший API контекста подвержен ошибкам и будет удален в будущей major версии. Он все еще работает для всех релизов 16.x, но в строгом режиме будет показано это предупреждение:
Изучите новую документацию по API контекста в данном разделе.
Strict Mode
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:
StrictMode currently helps with:
Additional functionality will be added with future releases of React.
Identifying unsafe lifecycles
As explained in this blog post, 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.
Warning about legacy string ref API usage
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 and so our official recommendation was to use the callback form instead.
React 16.3 added a third option that offers the convenience of a string ref without any of the downsides:
Since object refs were largely added as a replacement for string refs, strict mode now warns about usage of string refs.
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.
Warning about deprecated findDOMNode usage
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.
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.
You can also add a wrapper DOM node in your component and attach a ref directly to it.
In CSS, the display: contents attribute can be used if you don’t want the node to be part of the layout.
Detecting unexpected side effects
Conceptually, React does work in two phases:
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).
Render phase lifecycles include the following class component methods:
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.
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:
This only applies to development mode. Lifecycles will not be double-invoked in production mode.
For example, consider the following code:
At first glance, this code might not seem problematic. But if SharedApplicationState.recordEvent is not idempotent, 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.
By intentionally double-invoking methods like the component constructor, strict mode makes patterns like this easier to spot.
Starting with 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.
Detecting legacy context API
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:
Read the new context API documentation to help migrate to the new version.
Строгий режим — «use strict»
На протяжении долгого времени JavaScript развивался без проблем с обратной совместимостью. Новые функции добавлялись в язык, в то время как старая функциональность не менялась.
Преимуществом данного подхода было то, что существующий код продолжал работать. А недостатком – что любая ошибка или несовершенное решение, принятое создателями JavaScript, застревали в языке навсегда.
«use strict»
Позже мы изучим функции (способ группировки команд). Забегая вперёд, заметим, что вместо всего скрипта «use strict» можно поставить в начале большинства видов функций. Это позволяет включить строгий режим только в конкретной функции. Но обычно люди используют его для всего файла.
Проверьте, что «use strict» находится в первой исполняемой строке скрипта, иначе строгий режим может не включиться.
Здесь строгий режим не включён:
Над «use strict» могут быть записаны только комментарии.
Как только мы входим в строгий режим, отменить это невозможно.
Консоль браузера
В дальнейшем, когда вы будете использовать консоль браузера для тестирования функций, обратите внимание, что use strict по умолчанию в ней выключен.
Иногда, когда use strict имеет значение, вы можете получить неправильные результаты.
Можно использовать Shift + Enter для ввода нескольких строк и написать в верхней строке use strict :
В большинстве браузеров, включая Chrome и Firefox, это работает.
Всегда ли нужно использовать «use strict»?
Вопрос кажется риторическим, но это не так.
Кто-то посоветует начинать каждый скрипт с «use strict» … Но есть способ покруче.
Подытожим: пока очень желательно добавлять «use strict»; в начале ваших скриптов. Позже, когда весь ваш код будет состоять из классов и модулей, директиву можно будет опускать.
Пока мы узнали о use strict только в общих чертах.
В следующих главах, по мере расширения знаний о возможностях языка, мы яснее увидим отличия между строгим и стандартным режимом. К счастью, их не так много, и все они делают жизнь разработчика лучше.
Все примеры в этом учебнике подразумевают исполнение в строгом режиме, за исключением случаев (очень редких), когда оговорено иное.
Strict Mode
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:
StrictMode currently helps with:
Additional functionality will be added with future releases of React.
Identifying unsafe lifecycles
As explained in this blog post, 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.
Warning about legacy string ref API usage
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 and so our official recommendation was to use the callback form instead.
React 16.3 added a third option that offers the convenience of a string ref without any of the downsides:
Since object refs were largely added as a replacement for string refs, strict mode now warns about usage of string refs.
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.
Warning about deprecated findDOMNode usage
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.
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.
You can also add a wrapper DOM node in your component and attach a ref directly to it.
In CSS, the display: contents attribute can be used if you don’t want the node to be part of the layout.
Detecting unexpected side effects
Conceptually, React does work in two phases:
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).
Render phase lifecycles include the following class component methods:
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.
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:
This only applies to development mode. Lifecycles will not be double-invoked in production mode.
For example, consider the following code:
At first glance, this code might not seem problematic. But if SharedApplicationState.recordEvent is not idempotent, 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.
By intentionally double-invoking methods like the component constructor, strict mode makes patterns like this easier to spot.
Starting with 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.
Detecting legacy context API
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:
Read the new context API documentation to help migrate to the new version.
TL;DR — StrictMode is a feature added in version 16.3 and aimed to help us in finding potential problems in an application, at the moment especially for Concurrent-Mode which is React’s ability to concurrently render, suspend, and resume rendering trees in the background while remaining interactive.
Last month on twitter, I encountered a tweet by Sebastian Markbåge a React core team member asking developers if their app is running in :
The results blew my mind, 70% of us aren’t using it and never tried it!
At the moment, StrictMode helps us adapt our app to ⚡ Concurrent React ⚡ (Will be covered soon).
With React-Conf just around the corner (24–25/10), we still don’t know what the keynote by Tom Occhino and Yuzhi Zheng will be about, it looks like Concurrent mode might arrive sooner than we thought.
What is it?
Whats the motive?
Concurrent React is React’s new ability to concurrently render, suspend, and resume rendering trees in the background while remaining interactive (not destroying old trees while rendering the next).
Concurrent React’s main features are Time Slicing and a new Scheduler mechanism but it may also come with more features like React Suspense (for asynchronous data fetching) and Caching.
We know that Facebook are dogfooding Concurrent React for a while now in the new Facebook web (discovered in F8), iterating on bugs and working to get a stable version.
Just so you’ll get a glance of Concurrent mode’s power, an unstable example of Time-slicing was shown by Dan Abramov in his talk “Beyond React 16” and can be found here.
The example above shows Time slicing and scheduling, using it in Synchronous mode is the way React works now and using it in Asynchronous mode is an unstable example of Concurrent mode.
Concurrent mode is actually a different paradigm than the way we wrote JavaScript up until now, we should think of it more as a multi-threaded model, which schedules our work on a priority base.
As this paradigm is different, we need time and practice to get rid of old bad habits, trying out StrictMode is a great way to start.
How to use it?
Right after doing that you’ll be able to see warnings in your developer tools console.
At the moment, StrictMode helps with a few specific issues, we’ll cover them up and see how the warnings look like.
Detecting unexpected side effects:
While these phases will probably stay the same in Concurrent mode, the plan is to break the render phase into small pieces so we will be able to pause and resume the render phase so we won’t block the browser with long renders.
In fact, this means that React may trigger a render lifecycle more than once before committing the changes or even trigger a render without committing it (for example an error was caught or a higher order event happened).
So we can understand that the lifecycles in the render phase can be called more than one time, therefore it’s important that they will not contain side effects, if they will contain side effects, it may cause memory leaks and invalid state.
Identifying components with unsafe lifecycles:
Using unsafe lifecycles will produce this warning:
To help with refactoring your own code, there’s a code-mod that will rename all unsafe lifecycles and add the UNSAFE_ prefix, though keep in mind that this only removes the warning, it doesn’t remove the use of an unsafe lifecycle.
Warning about legacy string ref API usage:
Although being the convenient way, legacy string refs have several downsides. The main one was that they were resolved synchronously, which means they will create a problem when rendering will happen concurrently.
To fix this warning you have two options:
From version 16.3, Using createRef is the recommended way to create refs.
If you’re using an older version, the recommended way is a callback ref to make the ref creation “asynchronous”.
Lastly, you can also refactor your component to use Hooks, and use the useRef hook.
Using string refs will produce this warning:
Warning about deprecated findDOMNode usage:
Using deprecated findDOMNode will produce this warning:
Detecting legacy context API:
Using legacy context API will produce this warning:
Summing it up:
Hope I convinced you to add StrictMode to your app and that you’ve realised what’s coming up for us as React users.
If you have any questions, feel free to ask.
I’m here and also on twitter.
Thanks for reading!