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May 27, 2020. Debugging React. To debug the client side React code, we'll need to install the Debugger for Chrome extension. Note: This tutorial assumes you have the Chrome browser installed. There are also debugger extensions for the Edge and Firefox browsers. Open the Extensions view (⇧⌘X (Windows, Linux Ctrl+Shift+X)) and type 'chrome' in the search.
- Fast Refresh is a React Native feature that allows you to get near-instant feedback for changes in your React components. While debugging, it can help to have Fast Refresh enabled. Fast Refresh is enabled by default, and you can toggle 'Enable Fast Refresh' in the React Native developer menu.
- Jul 05, 2018.
- Feb 28, 2018.
- I'm using React.JS and when I do react-native run-android (with my device plugged in) I see a blank page. When I shake the device and select Debug JS Remotely from the option list I see the following screen. FYI: OS: Ubuntu 16.04 Node version is: v4.6.2 java version '1.8.0111' react': '15.4.1 react-native': '0.38.0.
Accessing the In-App Developer Menu
You can access the developer menu by shaking your device or by selecting 'Shake Gesture' inside the Hardware menu in the iOS Simulator. You can also use the ⌘D keyboard shortcut when your app is running in the iOS Simulator, or ⌘M when running in an Android emulator on Mac OS and Ctrl+M on Windows and Linux. Alternatively for Android, you can run the command adb shell input keyevent 82 to open the dev menu (82 being the Menu key code).
The Developer Menu is disabled in release (production) builds.
Enabling Fast Refresh
Fast Refresh is a React Native feature that allows you to get near-instant feedback for changes in your React components. While debugging, it can help to have Fast Refresh enabled. Fast Refresh is enabled by default, and you can toggle 'Enable Fast Refresh' in the React Native developer menu. When enabled, most of your edits should be visible within a second or two.
Enabling Keyboard Shortcuts
React Native supports a few keyboard shortcuts in the iOS Simulator. They are described below. To enable them, open the Hardware menu, select Keyboard, and make sure that 'Connect Hardware Keyboard' is checked.
LogBox
Errors and warnings in development builds are displayed in LogBox inside your app.
LogBox is automatically disabled in release (production) builds.
Console Errors and Warnings
Console errors and warnings are displayed as on-screen notifications with a red or yellow badge, and the number of errors or warning in the console respectively. To view a console error or warnings, tap the notification to view the full screen information about the log and to paginiate through all of the logs in the console.
These notifications can be hidden using LogBox.ignoreAllLogs(). This is useful when giving product demos, for example. Additionally, notifications can be hidden on a per-log basis via LogBox.ignoreLogs(). This is useful when there's a noisy warning that cannot be fixed, like those in a third-party dependency.
Ignore logs as a last resort and create a task to fix any logs that are ignored.
Unhandled Errors
Unhanded JavaScript errors such as undefined is not a function will automatically open a full screen LogBox error with the source of the error. These errors are dismissable and minimizable so that you can see the state of your app when these errors occur, but should always be addressed.
Syntax Errors
Syntax errors will automatically open a full screen LogBox error with the source of the syntax error. This error is not dismissable because it represents invalid JavaScript execution that must be fixed before continuing with your app. To dismiss these errors, fix the syntax error and either save to automatically dismiss (with Fast Refresh enabled) or cmd+r to reload (with Fast Refresh disabled).
Chrome Developer Tools
To debug the JavaScript code in Chrome, select 'Debug JS Remotely' from the Developer Menu. This will open a new tab at http://localhost:8081/debugger-ui.
Select Tools → Developer Tools from the Chrome Menu to open the Developer Tools. You may also access the DevTools using keyboard shortcuts (⌘⌥I on macOS, CtrlShiftI on Windows). You may also want to enable Pause On Caught Exceptions for a better debugging experience.
Note: the React Developer Tools Chrome extension does not work with React Native, but you can use its standalone version instead. Read this section to learn how.
Debugging using a custom JavaScript debugger
To use a custom JavaScript debugger in place of Chrome Developer Tools, set the REACT_DEBUGGER environment variable to a command that will start your custom debugger. You can then select 'Debug JS Remotely' from the Developer Menu to start debugging.
The debugger will receive a list of all project roots, separated by a space. For example, if you set REACT_DEBUGGER='node /path/to/launchDebugger.js --port 2345 --type ReactNative', then the command node /path/to/launchDebugger.js --port 2345 --type ReactNative /path/to/reactNative/app will be used to start your debugger.
Custom debugger commands executed this way should be short-lived processes, and they shouldn't produce more than 200 kilobytes of output.
Safari Developer Tools
You can use Safari to debug the iOS version of your app without having to enable 'Debug JS Remotely'.
- Enable Develop menu in Safari:
Preferences → Advanced → Select 'Show Develop menu in menu bar' - Select your app's JSContext:
Develop → Simulator → JSContext - Safari's Web Inspector should open which has a Console and a Debugger
While sourcemaps may not be enabled by default, you can follow this guide or video to enable them and set break points at the right places in the source code.
However, every time the app is reloaded (using live reload, or by manually reloading), a new JSContext is created. Choosing 'Automatically Show Web Inspectors for JSContexts' saves you from having to select the latest JSContext manually.
React Developer Tools
You can use the standalone version of React Developer Tools to debug the React component hierarchy. To use it, install the react-devtools package globally:
Note: Version 4 of react-devtools requires react-native version 0.62 or higher to work properly.
Now run react-devtools from the terminal to launch the standalone DevTools app:
It should connect to your simulator within a few seconds.
Note: if you prefer to avoid global installations, you can add react-devtools as a project dependency. Add the react-devtools package to your project using npm install --save-dev react-devtools, then add 'react-devtools': 'react-devtools' to the scripts section in your package.json, and then run npm run react-devtools from your project folder to open the DevTools.
Integration with React Native Inspector
Open the in-app developer menu and choose 'Toggle Inspector'. It will bring up an overlay that lets you tap on any UI element and see information about it:
However, when react-devtools is running, Inspector will enter a collapsed mode, and instead use the DevTools as primary UI. In this mode, clicking on something in the simulator will bring up the relevant components in the DevTools:
You can choose 'Toggle Inspector' in the same menu to exit this mode.
Inspecting Component Instances
When debugging JavaScript in Chrome, you can inspect the props and state of the React components in the browser console.
First, follow the instructions for debugging in Chrome to open the Chrome console.
Make sure that the dropdown in the top left corner of the Chrome console says debuggerWorker.js. This step is essential.
Then select a React component in React DevTools. There is a search box at the top that helps you find one by name. As soon as you select it, it will be available as $r in the Chrome console, letting you inspect its props, state, and instance properties.
Performance Monitor
You can enable a performance overlay to help you debug performance problems by selecting 'Perf Monitor' in the Developer Menu.
Debugging Application State
Reactotron is an open-source desktop app that allows you to inspect Redux or MobX-State-Tree application state as well as view custom logs, run custom commands such as resetting state, store and restore state snapshots, and other helpful debugging features for React Native apps.
You can view installation instructions in the README. If you're using Expo, here is an article detailing how to install on Expo.
Projects with Native Code Only
The following section only applies to projects with native code exposed. If you are using the managed expo-cli workflow, see the guide on ejecting to use this API.
Accessing console logs
You can display the console logs for an iOS or Android app by using the following commands in a terminal while the app is running:
You may also access these through Debug → Open System Log. in the iOS Simulator or by running adb logcat *:S ReactNative:V ReactNativeJS:V in a terminal while an Android app is running on a device or emulator.
If you're using Create React Native App or Expo CLI, console logs already appear in the same terminal output as the bundler.
Debugging on a device with Chrome Developer Tools
If you're using Create React Native App or Expo CLI, this is configured for you already.
On iOS devices, open the file RCTWebSocketExecutor.m and change 'localhost' to the IP address of your computer, then select 'Debug JS Remotely' from the Developer Menu.
On Android 5.0+ devices connected via USB, you can use the adb command line tool to setup port forwarding from the device to your computer:
adb reverse tcp:8081 tcp:8081
https://evilskyey.weebly.com/blog/microsoft-word-old-version-for-mac. Alternatively, select 'Dev Settings' from the Developer Menu, then update the 'Debug server host for device' setting to match the IP address of your computer.
If you run into any issues, it may be possible that one of your Chrome extensions is interacting in unexpected ways with the debugger. Try disabling all of your extensions and re-enabling them one-by-one until you find the problematic extension.
Debugging native code
When working with native code, such as when writing native modules, you can launch the app from Android Studio or Xcode and take advantage of the native debugging features (setting up breakpoints, etc.) as you would in case of building a standard native app.
React is a popular JavaScript library developed by Facebook for building web application user interfaces. The Visual Studio Code editor supports React.js IntelliSense and code navigation out of the box.
Welcome to React
We'll be using the create-react-appgenerator for this tutorial. To use the generator as well as run the React application server, you'll need Node.js JavaScript runtime and npm (Node.js package manager) installed. npm is included with Node.js which you can download and install from Node.js downloads.
Tip: To test that you have Node.js and npm correctly installed on your machine, you can type node --version and npm --version in a terminal or command prompt.
You can now create a new React application by typing:
where my-app is the name of the folder for your application. This may take a few minutes to create the React application and install its dependencies.
Note: If you've previously installed create-react-app globally via npm install -g create-react-app, we recommend you uninstall the package using npm uninstall -g create-react-app to ensure that npx always uses the latest version.
Let's quickly run our React application by navigating to the new folder and typing npm start to start the web server and open the application in a browser:
You should see the React logo and a link to 'Learn React' on http://localhost:3000 in your browser. We'll leave the web server running while we look at the application with VS Code.
To open your React application in VS Code, open another terminal or command prompt window, navigate to the my-app folder and type code .:
Markdown preview
In the File Explorer, one file you'll see is the application README.md Markdown file. This has lots of great information about the application and React in general. A nice way to review the README is by using the VS Code Markdown Preview. You can open the preview in either the current editor group (Markdown: Open Preview⇧⌘V (Windows, Linux Ctrl+Shift+V)) or in a new editor group to the side (Markdown: Open Preview to the Side⌘K V (Windows, Linux Ctrl+K V)). You'll get nice formatting, hyperlink navigation to headers, and syntax highlighting in code blocks.
Syntax highlighting and bracket matching
Now expand the src folder and select the index.js file. You'll notice that VS Code has syntax highlighting for the various source code elements and, if you put the cursor on a parenthesis, the matching bracket is also selected.
IntelliSense
As you start typing in index.js, you'll see smart suggestions or completions.
After you select a suggestion and type ., you see the types and methods on the object through IntelliSense.
VS Code uses the TypeScript language service for its JavaScript code intelligence and it has a feature called Automatic Type Acquisition (ATA). ATA pulls down the npm Type Declaration files (*.d.ts) for the npm modules referenced in the package.json.
If you select a method, you'll also get parameter help:
Go to Definition, Peek definition
Through the TypeScript language service, VS Code can also provide type definition information in the editor through Go to Definition (F12) or Peek Definition (⌥F12 (Windows Alt+F12, Linux Ctrl+Shift+F10)). Put the cursor over the App, right click and select Peek Definition. A Peek window will open showing the App definition from App.js.
Press Escape to close the Peek window.
Hello World!
Let's update the sample application to 'Hello World!'. Add the link to declare a new H1 header and replace the <App /> tag in ReactDOM.render with element.
Once you save the index.js file, the running instance of the server will update the web page and you'll see 'Hello World!'.
Tip: VS Code supports Auto Save, which by default saves your files after a delay. Check the Auto Save option in the File menu to turn on Auto Save or directly configure the files.autoSave user setting.
Debugging React
To debug the client side React code, we'll need to install the Debugger for Chrome extension.
Note: This tutorial assumes you have the Chrome browser installed. There are also debugger extensions for the Edge and Firefox browsers.
Open the Extensions view (⇧⌘X (Windows, Linux Ctrl+Shift+X)) and type 'chrome' in the search box. You'll see several extensions which reference Chrome.
Press the Install button for Debugger for Chrome.
Set a breakpoint
To set a breakpoint in index.js, click on the gutter to the left of the line numbers. This will set a breakpoint which will be visible as a red circle. Download minecraft free mac softonic.
Configure the Chrome debugger
We need to initially configure the debugger. To do so, go to the Run view (⇧⌘D (Windows, Linux Ctrl+Shift+D)) and click create a launch.json file to customize Run and Debug. Choose Chrome from the Select Environment drop-down list. This will create a launch.json file in a new .vscode folder in your project which includes a configuration to launch the website.
We need to make one change for our example: change the port of the url from 8080 to 3000. Your launch.json should look like this:
Ensure that your development server is running (npm start). Then press F5 or the green arrow to launch the debugger and open a new browser instance. The source code where the breakpoint is set runs on startup before the debugger was attached, so we won't hit the breakpoint until we refresh the web page. Refresh the page and you should hit your breakpoint.
You can step through your source code (F10), inspect variables such as element, and see the call stack of the client side React application.
The Debugger for Chrome extension README has lots of information on other configurations, working with sourcemaps, and troubleshooting. You can review it directly within VS Code from the Extensions view by clicking on the extension item and opening the Details view.
Live editing and debugging
If you are using webpack together with your React app, you can have a more efficient workflow by taking advantage of webpack's HMR mechanism which enables you to have live editing and debugging directly from VS Code. You can learn more in this Live edit and debug your React apps directly from VS Code blog post and the webpack Hot Module Replacement documentation.
Linting
Linters analyze your source code and can warn you about potential problems before you run your application. The JavaScript language services included with VS Code has syntax error checking support by default, which you can see in action in the Problems panel (View > Problems⇧⌘M (Windows, Linux Ctrl+Shift+M)).
Try making a small error in your React source code and you'll see a red squiggle and an error in the Problems panel.
Linters can provide more sophisticated analysis, enforcing coding conventions and detecting anti-patterns. A popular JavaScript linter is ESLint. ESLint, when combined with the ESLint VS Code extension, provides a great in-product linting experience.
First, install the ESLint command-line tool:
Then install the ESLint extension by going to the Extensions view and typing 'eslint'.
Once the ESLint extension is installed and VS Code reloaded, you'll want to create an ESLint configuration file, .eslintrc.js. You can create one using the extension's ESLint: Create ESLint configuration command from the Command Palette (⇧⌘P (Windows, Linux Ctrl+Shift+P)).
The command will prompt you to answer a series of questions in the Terminal panel. Take the defaults, and it will create a .eslintrc.js file in your project root that looks something like this:
ESLint will now analyze open files and shows a warning in index.js about 'App' being defined but never used.
You can modify the ESLint rules in the .eslintrc.js file.
Let's add an error rule for extra semi-colons:
Now when you mistakenly have multiple semicolons on a line, you'll see an error (red squiggle) in the editor and error entry in the Problems panel.
Popular Starter Kits
In this tutorial, we used the create-react-app generator to create a simple React application. There are lots of great samples and starter kits available to help build your first React application.
VS Code React Sample
This is a sample React application used for a demo at the 2016 //Build conference. The sample creates a simple TODO application and includes the source code for a Node.js Express server. It also shows how to use the Babel ES6 transpiler and then use webpack to bundle the site assets.
MERN
There is helpful VS Code-specific documentation at vscode-recipes which details setting up Node.js server debugging. VS Code also has great MongoDB support through the Azure Databases extension.
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TypeScript React
If you're curious about TypeScript and React, you can also create a TypeScript version of the create-react-app application. See the details at TypeScript-React-Starter on the TypeScript Quick Start site.
Angular
Angular is another popular web framework. If you'd like to see an example of Angular working with VS Code, check out the Chrome Debugging with Angular CLI recipe. It will walk you through creating an Angular application and configuring the launch.json file for the Debugger for Chrome extension.
Common questions
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Can I get IntelliSense within declarative JSX?
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Yes. For example, if you open the create-react-app project's App.js file, you can see IntelliSense within the React JSX in the render() method.