Errors in React.js include syntax errors, runtime errors, and errors related to the React library. So below is a list of the most common errors and problems that you will face while working in a React.js environment:
- Errors of Syntax
Unexpected Token: Commonly caused by missing or unnecessary commas, parentheses, brackets etc.
Invalid JSX: It contains non-concept elements like multiple root nodes which don’t have a single node as parent.
- Errors of Components
Uncaught ReferenceError: React is not defined: This occurs when files containing JSX code do not import React.
ReactDOM is not defined: Happens when ReactDOM is invoked without being imported.
Cannot read property ‘map’ of undefined: This could be the case when a non-existent array is being mapped over.
- State and Props problems
TypeError: Cannot read property ‘X’ of null/undefined: The missing property can either be a null or undefined value.
Props are read-only: Domesticating props as opposed to consequence management variables.
Cannot update a component while rendering another: Changes on either state or props are made during the rendering process.
- Errors of Lifecycle and Hooks
React Hook "useState"/"useEffect" is called conditionally: It is forbidden to call hooks conditionally, one must do so at the component’s top level.
Invalid hook call: Regarding the guarantees that hooks are called in function components, and custom hooks, but not outside or in the class components.
React Hook useEffect incomplete: Warning when there are undeclared dependencies in the dependency array of useEffect and the declaration is believed to be necessary.
- Rendering Problems
Nothing was swallowed from the render: This means that the components function fails to provide any jsx.
Invariant violation: Objects are not valid as a React child: Illogical rendering of objects including but not limited to rendering of strings.
A unique ‘key’ prop is required for each child in a list: Failure to furnish key attributes on the mapped child components to facilitate proper diffing.
- Routing Errors (React Router)
Invariant failed: You should not use
Cannot read property ‘push’ of undefined: Encounters concerning some navigation using history within React Router.
- Event Handling Errors
TypeError: Cannot read property ‘X’ of undefined: and in event handlers: as keywords are not correctly bound to this or arrow functions are being used.
Synthetic events reused: Trying to use some properties of a synthetic event after it has been pooled (for example: console.log(e.target.value) in an async function).
- Build and Deployment Errors
Module not found: Trying to import a file, or package that doesn’t exist, or was not correctly referenced.
Failed to compile: Showing a generic error with the message details during the build process.
This app requires JavaScript to be enabled in the browser: Problems related to configuration on the server-side or local client.
- Problems with Dependencies
Version mismatch: Old or inconsistent versions are available for the packages e.g. React and React DOM.
No such dependency: The errors brought about by packages that are supposed to be imported but are not installed e.g. npm install missing-package.
Babel or Webpack configuration issues: problems that are experienced when using bespoke build tools.
- State Management Problems
React context problem: Attempting to access a context without its matching Context.Provider.
Redux errors: An incorrectly configured store or misusing connect or useSelector hooks
State update on unmounted component: The state is being updated after a component has been removed from the DOM (often due to asynchronous actions).
- Strict Mode Errors
FindDOMNode is removed in StrictMode: Usage of findDOMNode feature when a component is wrapped in React.StrictMode.
Outdated lifecycle methods: There are warnings related to the componentWillMount, componentWillReceiveProps, componentWillUpdate.
- Compatibility
CORS CORS: Headers or server configuration problems when accessing an API.
Securities issues: Such warnings are mostly in relation to the use of dangerouslySetInnerHTML in an unjust manner.
- Issues of Performance
React performance warns: Rendering is inefficient because React.memo, useMemo or useCallback were not used where they had to be used.
Re-rendering issues: Over rendering as a result of a change in props or state which is not efficient.
- Miscellaneous
Hydration error: Mismatches occurred between content generated on the server and those generated on the client on a React app with SSR.
Unknown prop warning: Intrusion of unknown props on DOM elements, CustomProp, injected on a div for example.
Most often responding to such errors requires thorough scrutiny of the error message, examining the output, and grasping the cause of the error.
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