React: Hydration failed

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JavaScript

React: Hydration failed

React's server-rendered HTML did not match what the client rendered during hydration.

What it means

React's server-rendered HTML did not match what the client rendered during hydration.

Likely cause

Non-deterministic output such as Date.now()/Math.random(), browser-only values (window, localStorage) used during render, or invalid nested HTML.

How to fix it

Ensure server and client render identical markup: move browser-only or time-dependent logic into useEffect, or defer it with a mounted flag so it runs only after hydration.

Other JavaScript errors

TypeError: Cannot read properties of undefined Code tried to read a property of undefined (or null). TypeError: Cannot read properties of null Code tried to read a property or call a method on a value that is null. ReferenceError: x is not defined A variable or function name is used but was never declared in scope. TypeError: x is not a function Code tried to call something as a function that is not a function. SyntaxError: Unexpected token The JavaScript parser hit a character it did not expect. SyntaxError: Unexpected end of JSON input JSON.parse() received an empty or truncated string and could not finish parsing. RangeError: Maximum call stack size exceeded The call stack limit was exceeded. Uncaught (in promise) A promise rejected and nothing caught the error. Blocked by CORS policy: No 'Access-Control-Allow-Origin' The browser blocked a cross-origin request the server did not allow. TypeError: Failed to fetch fetch() could not complete the network request. TypeError: Assignment to constant variable Code tried to reassign a variable that was declared with const. TypeError: Cannot convert undefined or null to object An operation that expects an object (such as Object.keys or Object.assign) received null or undefined. SyntaxError: Cannot use import statement outside a module An ES import statement was used in a file that is not treated as an ES module. TypeError: x.map is not a function Code called .map() on a value that is not an array.

About Error Code Lookup

The Error Code Lookup is a searchable directory of more than 200 common error messages and codes across 12 categories: HTTP status codes, Windows error codes, browser and network errors, JavaScript, Python, PHP, Node.js and npm, Java, MySQL, Git, Linux shell errors and email/SMTP codes. Every entry is written in plain language for quick troubleshooting.

Each error is explained in three parts: what the message actually means, what usually causes it and concrete steps to fix it. That structure turns a cryptic string like ERR_CONNECTION_REFUSED or 0x80070005 into an actionable checklist instead of a guessing game.

You can filter the list by category, search by code, constant or message fragment, and copy any code with one click. Every error also has its own dedicated page with related errors from the same category, so you can bookmark or share a direct link to the exact fix.

How to use Error Code Lookup

  1. Type the error code or message into the search box — for example 404, ERR_NAME_NOT_RESOLVED or 0x80070005.
  2. Narrow the results with the category filter (HTTP, Windows, JavaScript, MySQL and so on) if the search returns too many entries.
  3. Open the matching entry to read what it means, its likely cause and how to fix it.
  4. Use the copy button to grab the exact code for a bug report or support ticket.
  5. Check the related errors section for other errors from the same category.

Frequently asked questions

It covers 12 categories: HTTP status codes, Windows system errors, browser and network errors, JavaScript, Python, PHP, Node.js and npm, Java, MySQL, Git, Linux shell errors and email/SMTP reply codes — more than 200 entries in total.

Search with fewer words. The shortest unique part works best: just the numeric code (“404”, “0x80070005”) or the constant name (“ERR_NAME_NOT_RESOLVED”) rather than the full sentence your system printed.

Yes. Each entry has a dedicated detail page explaining the meaning, likely cause and fix, plus related errors from the same category. You can bookmark it or send the link directly to a colleague or a support ticket.

The fixes describe the standard, widely accepted resolution for each error. Still, read the likely-cause section first — the same code can have several triggers, and matching the cause to your situation tells you which fix applies.

Yes, it is completely free. You can search, browse the category directory and open as many error detail pages as you need without an account.
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