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JavaScript errors are specific. Each one tells you exactly what went wrong if you know how to read it. Most developers waste time debugging because they treat error messages as obstacles rather than instructions. This guide translates the errors you are most likely to encounter into plain language and tells you what to change.
ReferenceError: document is not defined
This error appears when code that accesses the browser’s document object runs in a Node.js environment, which has no document. Common cause: a module written for the browser is imported in a server-side rendering context or in a Node script.
Fix: guard browser-specific code with a check — if (typeof document !== ‘undefined’) — or move the code into a lifecycle hook that only runs client-side, such as React’s useEffect or Next.js’s dynamic import with ssr: false.
ReferenceError: require is not defined
require() is Node.js’s module system. This error appears when require() is used in a browser context or in a project configured to use ES modules (where import/export is the correct syntax). It also appears when an ES module file uses require() without a CommonJS configuration.
Fix: replace require() with import statements if your project uses ES modules. If you need CommonJS in a Node project configured as ES modules, rename the file to .cjs or add type: commonjs to the package.json.
TypeError: Cannot Read Properties of Null
This error means you attempted to access a property on a value that is null. The pattern: let el = document.getElementById(‘myId’); el.innerHTML = ‘hello’; — if the element does not exist, getElementById returns null and the property access throws.
Fix: check for null before accessing properties — if (el) el.innerHTML = ‘hello’. For React, this error often appears when accessing DOM elements before the component mounts: move the access into a useEffect to guarantee the element exists.
SyntaxError: Unexpected End of Input
This error means the JavaScript parser reached the end of the file while expecting more code. Usually caused by a missing closing bracket, brace, or parenthesis. Also caused by a JSON.parse() call on an empty or truncated string.
Fix: use your editor’s bracket matching to find unclosed braces. For JSON parse errors, check the source of the string — log it before parsing to confirm it is valid, non-empty JSON.
TypeError: X is not a function
This error means you called something as a function that is not a function. Common causes: a variable shadows a function with the same name, a method is called on the wrong type, or an async function’s return value is used synchronously.
Fix: console.log the value before calling it to verify its type. Check for naming collisions. If the function is async, await the result before calling methods on it.
getElementById Returns Null
getElementById returns null when no element with the given ID exists in the DOM at the time the script runs. This typically happens when a script in the head runs before the body elements are parsed.
Fix: move scripts to the end of the body, use the defer attribute, or wrap the code in a DOMContentLoaded event listener to ensure the DOM is fully built before the script runs.
Frequently Asked Questions
How do I debug JavaScript errors in production? Use an error monitoring service like Sentry or Datadog. These capture errors with stack traces from real users in production, which browser DevTools only shows during active development sessions.
Why does my JavaScript work in Chrome but not Safari? Browser compatibility differences. Check caniuse.com for the feature causing the error. Babel or transpilation tools can compile modern JavaScript syntax to versions supported by older browsers.
What does ‘Uncaught’ in a JavaScript error mean? Uncaught means the error was not handled by a try/catch block or a Promise rejection handler. It propagated up to the global error handler and halted execution. Add error handling around the operation that threw.
How do I fix JavaScript errors in a WordPress plugin? Open Chrome DevTools console and identify the specific error. Check which plugin script is responsible by looking at the source file in the error message. The error usually traces to a conflict between plugin scripts, a jQuery dependency issue, or a plugin trying to access a DOM element that does not exist on the current page.