⚠️ Error Reference

Web Error Reference

HTTP status codes, JavaScript errors, console warnings, and CSS rendering issues — with causes, examples, and fixes. Stop Googling the same errors twice.

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HTTP Status Codes

Server responses and what they mean

The request succeeded. The response body contains the requested resource.

301 Moved Permanently

The resource has been permanently moved. Browsers and search engines follow the Location header.

400 Bad Request

The server couldn't understand the request due to invalid syntax. Check your request format and headers.

401 Unauthorized

Authentication is required and has not been provided or is invalid. Differs from 403 Forbidden.

The server understands the request but refuses to authorise it. The client's identity doesn't matter.

The server can't find the requested resource. The URL may be wrong or the resource deleted.

500 Internal Server Error

The server encountered an unexpected condition. Check server logs for the specific cause.

503 Service Unavailable

The server is temporarily unable to handle the request — overloaded or down for maintenance.

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JavaScript Errors

Common runtime and type errors explained

TypeError: undefined is not a function

You're calling something as a function that isn't one. Usually caused by a typo or wrong variable reference.

Cannot read properties of null

Accessing a property on null or undefined. The most common JS error — usually a timing or async issue.

Cross-Origin Resource Sharing policy blocked the request. Requires server-side Access-Control headers.

ReferenceError: x is not defined

Using a variable that hasn't been declared. Check scope, spelling, and import statements.

SyntaxError: Unexpected token

The JavaScript parser hit something it didn't expect. Usually a missing bracket, comma, or semicolon.

UnhandledPromiseRejection

A Promise was rejected with no .catch() or try/catch handler. Always handle async errors explicitly.

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CSS Issues

Common rendering and specificity problems

Styles Not Applying

A more specific CSS rule is overriding yours. Understand specificity scoring and how to resolve conflicts.

z-index Not Working

z-index only works on positioned elements. Stacking contexts created by opacity, transforms, or filters isolate z-index scope.

Flex Items Stretching

align-items defaults to stretch, making flex children fill the cross-axis. Set align-items: flex-start to prevent this.

A Developer's Guide to Web Errors

Every developer encounters the same set of errors repeatedly throughout their career. HTTP 404s, CORS errors, JavaScript TypeErrors, and CSS specificity conflicts are so common that recognising them on sight and knowing the fix instantly is one of the most valuable developer skills to build.

How to Read HTTP Status Codes

HTTP status codes are grouped by their first digit: 1xx informational, 2xx success, 3xx redirection, 4xx client errors, 5xx server errors. When you see an error, the first digit immediately tells you whether the problem is on the client side (4xx — your request was wrong) or server side (5xx — the server failed).

Debugging JavaScript Errors

The browser console is your primary debugging tool. Always read the full error message, including the file name and line number. Use the call stack to trace where the error originated. For async errors, check whether you're properly handling Promise rejections.