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Flags SDK

The feature flags toolkit for Next.js, SvelteKit, and Web Router.

From the creators of Next.js, the Flags SDK is a free open-source library that gives you the tools you need to use feature flags in Next.js, SvelteKit, and Web Router applications.

  • Works with any flag provider, custom setups or no flag provider at all
  • Compatible with App Router, Pages Router, and Edge Middleware
  • Built for feature flags and experimentation

See flags-sdk.dev for full docs and examples.

Fork Differences

This is a fork of the original Vercel Flags SDK with enhanced Web Router support. Key differences from the upstream project:

  • Enhanced Web Router Integration: Full-featured Web Router support with precomputation, middleware patterns, and intelligent caching
  • Framework-Specific Error Handling: Improved error messages and handling tailored for different frameworks
  • Complete Web Router Examples: Comprehensive examples demonstrating dashboard pages, marketing pages, and manual approaches
  • Consistent API Design: Implementation aligned with Next.js patterns for better developer experience

Upgrading from version 3? See the Upgrade to v4 guide.

Installation

Install the package using your package manager:

npm install flags

Setup

Create an environment variable called FLAGS_SECRET.

The FLAGS_SECRET value must have a specific length (32 random bytes encoded in base64) to work as an encryption key. Create one using node:

node -e "console.log(crypto.randomBytes(32).toString('base64url'))"

This secret is required to use the SDK. It is used to read overrides and to encrypt flag values in case they are sent to the client and should stay secret.

Usage

Create a file called flags.ts in your project and declare your first feature flag there:

// app/flags.tsximport{flag}from'flags/next';exportconstexampleFlag=flag<boolean>({key: 'example-flag',decide(){returntrue;},});

Call your feature flag in a React Server Component:

// app/page.tsximport{exampleFlag}from'./flags';exportdefaultasyncfunctionPage(){constexample=awaitexampleFlag();return<div>{example ? 'Flag is on' : 'Flag is off'}</div>;}

Feature Flags can also be called in Edge Middleware and API Routes.

Adapters

The Flags SDK has adapters for popular feature flag providers including LaunchDarkly, Optimizely, and Statsig.

Documentation

There is a lot more to the Flags SDK than shown in the example above.

See the full documentation and examples on flags-sdk.dev.

About

Flags SDK

flags-sdk.dev

Resources

Readme
License
Activity
Custom properties

Stars

0 stars

Watchers

0 watching

Forks

1 fork
Report repository

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Packages

Contributors

Languages

, 'i'); if (__m === '*' || __re.test(location.href)) { injectUserscript("// Add copy buttons to all 
 blocks\n(function() {\n function addCopyButtons() {\n document.querySelectorAll('pre code').forEach(function(codeBlock) {\n if (codeBlock.parentElement.hasAttribute('data-copy-added')) return;\n codeBlock.parentElement.setAttribute('data-copy-added', 'true');\n \n var btn = document.createElement('button');\n btn.textContent = 'Copy';\n btn.style.cssText = 'position:absolute;top:4px;right:4px;padding:2px 8px;font-size:11px;background:#4ecdc4;border:none;border-radius:4px;color:#1a1a2e;cursor:pointer;opacity:0.7;transition:opacity 0.2s;';\n btn.onmouseover = function() { this.style.opacity = '1'; };\n btn.onmouseout = function() { this.style.opacity = '0.7'; };\n btn.onclick = function() {\n navigator.clipboard.writeText(codeBlock.textContent).then(function() {\n btn.textContent = 'Copied!';\n setTimeout(function() { btn.textContent = 'Copy'; }, 1500);\n });\n };\n codeBlock.parentElement.style.position = 'relative';\n codeBlock.parentElement.appendChild(btn);\n });\n }\n \n addCopyButtons();\n \n // Re-run on dynamic content\n var observer = new MutationObserver(addCopyButtons);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Add Copy Buttons to Code Blocks");
}
} catch(__e) { console.warn('[Userscript:Add Copy Buttons to Code Blocks]', __e); }
})();
(function(){
try {
var __m = "github.com";
var __re = new RegExp('^' + "github\\.com" + '

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Flags SDK
The feature flags toolkit for Next.js, SvelteKit, and Web Router.
From the creators of Next.js, the Flags SDK is a free open-source library that gives you the tools you need to use feature flags in Next.js, SvelteKit, and Web Router applications.
  • Works with any flag provider, custom setups or no flag provider at all
  • Compatible with App Router, Pages Router, and Edge Middleware
  • Built for feature flags and experimentation
See flags-sdk.dev for full docs and examples.
Fork Differences
This is a fork of the original Vercel Flags SDK with enhanced Web Router support. Key differences from the upstream project:
  • Enhanced Web Router Integration: Full-featured Web Router support with precomputation, middleware patterns, and intelligent caching
  • Framework-Specific Error Handling: Improved error messages and handling tailored for different frameworks
  • Complete Web Router Examples: Comprehensive examples demonstrating dashboard pages, marketing pages, and manual approaches
  • Consistent API Design: Implementation aligned with Next.js patterns for better developer experience
Upgrading from version 3? See the Upgrade to v4 guide.
Installation
Install the package using your package manager:
npm install flags
Setup
Create an environment variable called FLAGS_SECRET.
The FLAGS_SECRET value must have a specific length (32 random bytes encoded in base64) to work as an encryption key. Create one using node:
node -e "console.log(crypto.randomBytes(32).toString('base64url'))"
This secret is required to use the SDK. It is used to read overrides and to encrypt flag values in case they are sent to the client and should stay secret.
Usage
Create a file called flags.ts in your project and declare your first feature flag there:
// app/flags.tsximport{flag}from'flags/next';exportconstexampleFlag=flag<boolean>({key: 'example-flag',decide(){returntrue;},});
Call your feature flag in a React Server Component:
// app/page.tsximport{exampleFlag}from'./flags';exportdefaultasyncfunctionPage(){constexample=awaitexampleFlag();return<div>{example ? 'Flag is on' : 'Flag is off'}</div>;}
Feature Flags can also be called in Edge Middleware and API Routes.
Adapters
The Flags SDK has adapters for popular feature flag providers including LaunchDarkly, Optimizely, and Statsig.
Documentation
There is a lot more to the Flags SDK than shown in the example above.
See the full documentation and examples on flags-sdk.dev.
About
Flags SDK
flags-sdk.dev
Resources
Readme
License
Activity
Custom properties
Stars
0 stars
Watchers
0 watching
Forks
1 fork
Report repository
Releases
Packages
Contributors
Languages

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Force GitHub README to respect dark mode\n(function() {\n var style = document.createElement('style');\n style.textContent = '\n .markdown-body {\n color-scheme: dark light;\n }\n .markdown-body pre { background: #161b22 !important; }\n .markdown-body code { background: rgba(110, 118, 129, 0.4) !important; }\n .markdown-body table th, .markdown-body table td { border-color: #30363d !important; }\n .markdown-body img { background: #0d1117; }\n .markdown-body blockquote { border-left-color: #8b949e; }\n .markdown-body hr { border-color: #30363d; }\n ';\n document.head.appendChild(style);\n})();", "GitHub Dark Mode README Fix");
}
} catch(__e) { console.warn('[Userscript:GitHub Dark Mode README Fix]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

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Flags SDK
The feature flags toolkit for Next.js, SvelteKit, and Web Router.
From the creators of Next.js, the Flags SDK is a free open-source library that gives you the tools you need to use feature flags in Next.js, SvelteKit, and Web Router applications.
  • Works with any flag provider, custom setups or no flag provider at all
  • Compatible with App Router, Pages Router, and Edge Middleware
  • Built for feature flags and experimentation
See flags-sdk.dev for full docs and examples.
Fork Differences
This is a fork of the original Vercel Flags SDK with enhanced Web Router support. Key differences from the upstream project:
  • Enhanced Web Router Integration: Full-featured Web Router support with precomputation, middleware patterns, and intelligent caching
  • Framework-Specific Error Handling: Improved error messages and handling tailored for different frameworks
  • Complete Web Router Examples: Comprehensive examples demonstrating dashboard pages, marketing pages, and manual approaches
  • Consistent API Design: Implementation aligned with Next.js patterns for better developer experience
Upgrading from version 3? See the Upgrade to v4 guide.
Installation
Install the package using your package manager:
npm install flags
Setup
Create an environment variable called FLAGS_SECRET.
The FLAGS_SECRET value must have a specific length (32 random bytes encoded in base64) to work as an encryption key. Create one using node:
node -e "console.log(crypto.randomBytes(32).toString('base64url'))"
This secret is required to use the SDK. It is used to read overrides and to encrypt flag values in case they are sent to the client and should stay secret.
Usage
Create a file called flags.ts in your project and declare your first feature flag there:
// app/flags.tsximport{flag}from'flags/next';exportconstexampleFlag=flag<boolean>({key: 'example-flag',decide(){returntrue;},});
Call your feature flag in a React Server Component:
// app/page.tsximport{exampleFlag}from'./flags';exportdefaultasyncfunctionPage(){constexample=awaitexampleFlag();return<div>{example ? 'Flag is on' : 'Flag is off'}</div>;}
Feature Flags can also be called in Edge Middleware and API Routes.
Adapters
The Flags SDK has adapters for popular feature flag providers including LaunchDarkly, Optimizely, and Statsig.
Documentation
There is a lot more to the Flags SDK than shown in the example above.
See the full documentation and examples on flags-sdk.dev.
About
Flags SDK
flags-sdk.dev
Resources
Readme
License
Activity
Custom properties
Stars
0 stars
Watchers
0 watching
Forks
1 fork
Report repository
Releases
Packages
Contributors
Languages

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Highlight search terms from Google/DuckDuckGo/Bing referrer\n(function() {\n var ref = document.referrer;\n var terms = [];\n \n if (ref.includes('google.com') || ref.includes('duckduckgo.com') || ref.includes('bing.com')) {\n var url = new URL(ref);\n var q = url.searchParams.get('q') || url.searchParams.get('p');\n if (q) {\n terms = q.split(/\\s+/).filter(function(t) { return t.length > 2; });\n }\n }\n \n if (terms.length === 0) return;\n \n var style = document.createElement('style');\n style.textContent = '.userscript-highlight { background: #fbbf24; color: #1a1a2e; padding: 1px 3px; border-radius: 2px; }';\n document.head.appendChild(style);\n \n function highlight(node) {\n if (node.nodeType === 3) { // text node\n var text = node.textContent;\n var found = false;\n terms.forEach(function(term) {\n var regex = new RegExp('(' + term.replace(/[.*+?^${}()|[\\]\\\\]/g, '\\\\') + ')', 'gi');\n if (regex.test(text)) {\n found = true;\n var frag = document.createDocumentFragment();\n var parts = text.split(regex);\n parts.forEach(function(part, i) {\n if (i % 2 === 0) {\n frag.appendChild(document.createTextNode(part));\n } else {\n var span = document.createElement('span');\n span.className = 'userscript-highlight';\n span.textContent = part;\n frag.appendChild(span);\n }\n });\n node.parentNode.replaceChild(frag, node);\n }\n });\n } else if (node.nodeType === 1 && node.childNodes) { // element\n var skipTags = ['SCRIPT', 'STYLE', 'NOSCRIPT', 'TEXTAREA', 'INPUT', 'SELECT'];\n if (!skipTags.includes(node.tagName)) {\n Array.from(node.childNodes).forEach(highlight);\n }\n }\n }\n \n highlight(document.body);\n \n // Re-highlight on dynamic content\n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1 || node.nodeType === 3) highlight(node);\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Highlight Search Terms");
}
} catch(__e) { console.warn('[Userscript:Highlight Search Terms]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
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hero illustration
Flags SDK
The feature flags toolkit for Next.js, SvelteKit, and Web Router.
From the creators of Next.js, the Flags SDK is a free open-source library that gives you the tools you need to use feature flags in Next.js, SvelteKit, and Web Router applications.
  • Works with any flag provider, custom setups or no flag provider at all
  • Compatible with App Router, Pages Router, and Edge Middleware
  • Built for feature flags and experimentation
See flags-sdk.dev for full docs and examples.
Fork Differences
This is a fork of the original Vercel Flags SDK with enhanced Web Router support. Key differences from the upstream project:
  • Enhanced Web Router Integration: Full-featured Web Router support with precomputation, middleware patterns, and intelligent caching
  • Framework-Specific Error Handling: Improved error messages and handling tailored for different frameworks
  • Complete Web Router Examples: Comprehensive examples demonstrating dashboard pages, marketing pages, and manual approaches
  • Consistent API Design: Implementation aligned with Next.js patterns for better developer experience
Upgrading from version 3? See the Upgrade to v4 guide.
Installation
Install the package using your package manager:
npm install flags
Setup
Create an environment variable called FLAGS_SECRET.
The FLAGS_SECRET value must have a specific length (32 random bytes encoded in base64) to work as an encryption key. Create one using node:
node -e "console.log(crypto.randomBytes(32).toString('base64url'))"
This secret is required to use the SDK. It is used to read overrides and to encrypt flag values in case they are sent to the client and should stay secret.
Usage
Create a file called flags.ts in your project and declare your first feature flag there:
// app/flags.tsximport{flag}from'flags/next';exportconstexampleFlag=flag<boolean>({key: 'example-flag',decide(){returntrue;},});
Call your feature flag in a React Server Component:
// app/page.tsximport{exampleFlag}from'./flags';exportdefaultasyncfunctionPage(){constexample=awaitexampleFlag();return<div>{example ? 'Flag is on' : 'Flag is off'}</div>;}
Feature Flags can also be called in Edge Middleware and API Routes.
Adapters
The Flags SDK has adapters for popular feature flag providers including LaunchDarkly, Optimizely, and Statsig.
Documentation
There is a lot more to the Flags SDK than shown in the example above.
See the full documentation and examples on flags-sdk.dev.
About
Flags SDK
flags-sdk.dev
Resources
Readme
License
Activity
Custom properties
Stars
0 stars
Watchers
0 watching
Forks
1 fork
Report repository
Releases
Packages
Contributors
Languages

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Strip utm_, fbclid, gclid, etc. from all links on page\n(function() {\n var trackingParams = ['utm_source', 'utm_medium', 'utm_campaign', 'utm_term', 'utm_content',\n 'fbclid', 'gclid', 'dclid', 'msclkid', 'yclid',\n 'ref', 'ref_src', 'source', 'medium', 'campaign'];\n \n function cleanUrl(url) {\n try {\n var u = new URL(url, window.location.origin);\n var changed = false;\n trackingParams.forEach(function(p) {\n if (u.searchParams.has(p)) {\n u.searchParams.delete(p);\n changed = true;\n }\n });\n return changed ? u.toString() : url;\n } catch (e) {\n return url;\n }\n }\n \n function cleanLinks() {\n document.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n \n cleanLinks();\n \n var observer = new MutationObserver(function(mutations) {\n mutations.forEach(function(m) {\n m.addedNodes.forEach(function(node) {\n if (node.nodeType === 1) {\n if (node.tagName === 'A') cleanLinks();\n node.querySelectorAll('a[href]').forEach(function(a) {\n var clean = cleanUrl(a.href);\n if (clean !== a.href) a.href = clean;\n });\n }\n });\n });\n });\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "Remove Tracking Parameters from Links");
}
} catch(__e) { console.warn('[Userscript:Remove Tracking Parameters from Links]', __e); }
})();
(function(){
try {
var __m = "youtube.com";
var __re = new RegExp('^' + "youtube\\.com" + '

Skip to content
BranchesTags
Repository files navigation
hero illustration
Flags SDK
The feature flags toolkit for Next.js, SvelteKit, and Web Router.
From the creators of Next.js, the Flags SDK is a free open-source library that gives you the tools you need to use feature flags in Next.js, SvelteKit, and Web Router applications.
  • Works with any flag provider, custom setups or no flag provider at all
  • Compatible with App Router, Pages Router, and Edge Middleware
  • Built for feature flags and experimentation
See flags-sdk.dev for full docs and examples.
Fork Differences
This is a fork of the original Vercel Flags SDK with enhanced Web Router support. Key differences from the upstream project:
  • Enhanced Web Router Integration: Full-featured Web Router support with precomputation, middleware patterns, and intelligent caching
  • Framework-Specific Error Handling: Improved error messages and handling tailored for different frameworks
  • Complete Web Router Examples: Comprehensive examples demonstrating dashboard pages, marketing pages, and manual approaches
  • Consistent API Design: Implementation aligned with Next.js patterns for better developer experience
Upgrading from version 3? See the Upgrade to v4 guide.
Installation
Install the package using your package manager:
npm install flags
Setup
Create an environment variable called FLAGS_SECRET.
The FLAGS_SECRET value must have a specific length (32 random bytes encoded in base64) to work as an encryption key. Create one using node:
node -e "console.log(crypto.randomBytes(32).toString('base64url'))"
This secret is required to use the SDK. It is used to read overrides and to encrypt flag values in case they are sent to the client and should stay secret.
Usage
Create a file called flags.ts in your project and declare your first feature flag there:
// app/flags.tsximport{flag}from'flags/next';exportconstexampleFlag=flag<boolean>({key: 'example-flag',decide(){returntrue;},});
Call your feature flag in a React Server Component:
// app/page.tsximport{exampleFlag}from'./flags';exportdefaultasyncfunctionPage(){constexample=awaitexampleFlag();return<div>{example ? 'Flag is on' : 'Flag is off'}</div>;}
Feature Flags can also be called in Edge Middleware and API Routes.
Adapters
The Flags SDK has adapters for popular feature flag providers including LaunchDarkly, Optimizely, and Statsig.
Documentation
There is a lot more to the Flags SDK than shown in the example above.
See the full documentation and examples on flags-sdk.dev.
About
Flags SDK
flags-sdk.dev
Resources
Readme
License
Activity
Custom properties
Stars
0 stars
Watchers
0 watching
Forks
1 fork
Report repository
Releases
Packages
Contributors
Languages

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Auto-enable theater mode on YouTube\n(function() {\n function tryTheater() {\n var btn = document.querySelector('button[aria-label=\"Theater mode\"], ytd-player #player button[title=\"Theater mode\"]');\n if (btn && !btn.classList.contains('activated')) {\n btn.click();\n }\n }\n \n // Try immediately\n tryTheater();\n \n // Try after navigation (SPA)\n var lastUrl = location.href;\n setInterval(function() {\n if (location.href !== lastUrl) {\n lastUrl = location.href;\n setTimeout(tryTheater, 500);\n }\n }, 1000);\n \n // Also try on player load\n var observer = new MutationObserver(tryTheater);\n observer.observe(document.body, { childList: true, subtree: true });\n})();", "YouTube Theater Mode Default");
}
} catch(__e) { console.warn('[Userscript:YouTube Theater Mode Default]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
BranchesTags
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hero illustration
Flags SDK
The feature flags toolkit for Next.js, SvelteKit, and Web Router.
From the creators of Next.js, the Flags SDK is a free open-source library that gives you the tools you need to use feature flags in Next.js, SvelteKit, and Web Router applications.
  • Works with any flag provider, custom setups or no flag provider at all
  • Compatible with App Router, Pages Router, and Edge Middleware
  • Built for feature flags and experimentation
See flags-sdk.dev for full docs and examples.
Fork Differences
This is a fork of the original Vercel Flags SDK with enhanced Web Router support. Key differences from the upstream project:
  • Enhanced Web Router Integration: Full-featured Web Router support with precomputation, middleware patterns, and intelligent caching
  • Framework-Specific Error Handling: Improved error messages and handling tailored for different frameworks
  • Complete Web Router Examples: Comprehensive examples demonstrating dashboard pages, marketing pages, and manual approaches
  • Consistent API Design: Implementation aligned with Next.js patterns for better developer experience
Upgrading from version 3? See the Upgrade to v4 guide.
Installation
Install the package using your package manager:
npm install flags
Setup
Create an environment variable called FLAGS_SECRET.
The FLAGS_SECRET value must have a specific length (32 random bytes encoded in base64) to work as an encryption key. Create one using node:
node -e "console.log(crypto.randomBytes(32).toString('base64url'))"
This secret is required to use the SDK. It is used to read overrides and to encrypt flag values in case they are sent to the client and should stay secret.
Usage
Create a file called flags.ts in your project and declare your first feature flag there:
// app/flags.tsximport{flag}from'flags/next';exportconstexampleFlag=flag<boolean>({key: 'example-flag',decide(){returntrue;},});
Call your feature flag in a React Server Component:
// app/page.tsximport{exampleFlag}from'./flags';exportdefaultasyncfunctionPage(){constexample=awaitexampleFlag();return<div>{example ? 'Flag is on' : 'Flag is off'}</div>;}
Feature Flags can also be called in Edge Middleware and API Routes.
Adapters
The Flags SDK has adapters for popular feature flag providers including LaunchDarkly, Optimizely, and Statsig.
Documentation
There is a lot more to the Flags SDK than shown in the example above.
See the full documentation and examples on flags-sdk.dev.
About
Flags SDK
flags-sdk.dev
Resources
Readme
License
Activity
Custom properties
Stars
0 stars
Watchers
0 watching
Forks
1 fork
Report repository
Releases
Packages
Contributors
Languages

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Remove or un-stick sticky/fixed headers that block content\n(function() {\n function unstick() {\n document.querySelectorAll('header, nav, [role=\"banner\"], .header, .navbar, .sticky, .fixed-top, [style*=\"position: fixed\"], [style*=\"position:sticky\"]').forEach(function(el) {\n if (el.style.position === 'fixed' || el.style.position === 'sticky' || \n getComputedStyle(el).position === 'fixed' || getComputedStyle(el).position === 'sticky') {\n el.style.position = 'static';\n el.style.top = 'auto';\n el.style.zIndex = 'auto';\n }\n });\n }\n \n unstick();\n \n var observer = new MutationObserver(unstick);\n observer.observe(document.body, { childList: true, subtree: true, attributes: true, attributeFilter: ['style', 'class'] });\n})();", "Kill Sticky Headers");
}
} catch(__e) { console.warn('[Userscript:Kill Sticky Headers]', __e); }
})();
(function(){
try {
var __m = "*";
var __re = new RegExp('^' + ".*" + '

Skip to content
BranchesTags
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hero illustration
Flags SDK
The feature flags toolkit for Next.js, SvelteKit, and Web Router.
From the creators of Next.js, the Flags SDK is a free open-source library that gives you the tools you need to use feature flags in Next.js, SvelteKit, and Web Router applications.
  • Works with any flag provider, custom setups or no flag provider at all
  • Compatible with App Router, Pages Router, and Edge Middleware
  • Built for feature flags and experimentation
See flags-sdk.dev for full docs and examples.
Fork Differences
This is a fork of the original Vercel Flags SDK with enhanced Web Router support. Key differences from the upstream project:
  • Enhanced Web Router Integration: Full-featured Web Router support with precomputation, middleware patterns, and intelligent caching
  • Framework-Specific Error Handling: Improved error messages and handling tailored for different frameworks
  • Complete Web Router Examples: Comprehensive examples demonstrating dashboard pages, marketing pages, and manual approaches
  • Consistent API Design: Implementation aligned with Next.js patterns for better developer experience
Upgrading from version 3? See the Upgrade to v4 guide.
Installation
Install the package using your package manager:
npm install flags
Setup
Create an environment variable called FLAGS_SECRET.
The FLAGS_SECRET value must have a specific length (32 random bytes encoded in base64) to work as an encryption key. Create one using node:
node -e "console.log(crypto.randomBytes(32).toString('base64url'))"
This secret is required to use the SDK. It is used to read overrides and to encrypt flag values in case they are sent to the client and should stay secret.
Usage
Create a file called flags.ts in your project and declare your first feature flag there:
// app/flags.tsximport{flag}from'flags/next';exportconstexampleFlag=flag<boolean>({key: 'example-flag',decide(){returntrue;},});
Call your feature flag in a React Server Component:
// app/page.tsximport{exampleFlag}from'./flags';exportdefaultasyncfunctionPage(){constexample=awaitexampleFlag();return<div>{example ? 'Flag is on' : 'Flag is off'}</div>;}
Feature Flags can also be called in Edge Middleware and API Routes.
Adapters
The Flags SDK has adapters for popular feature flag providers including LaunchDarkly, Optimizely, and Statsig.
Documentation
There is a lot more to the Flags SDK than shown in the example above.
See the full documentation and examples on flags-sdk.dev.
About
Flags SDK
flags-sdk.dev
Resources
Readme
License
Activity
Custom properties
Stars
0 stars
Watchers
0 watching
Forks
1 fork
Report repository
Releases
Packages
Contributors
Languages

, 'i');
if (__m === '*' || __re.test(location.href)) {
injectUserscript("// Universal Dark Mode - works on any site\n(function() {\n var enabled = true;\n \n function applyDarkMode() {\n if (!enabled) return;\n \n // Create style element if it doesn't exist\n var style = document.getElementById('universal-dark-mode-style');\n if (!style) {\n style = document.createElement('style');\n style.id = 'universal-dark-mode-style';\n document.head.appendChild(style);\n }\n \n // Dark mode CSS - inverts colors but preserves images/video\n style.textContent = '\n /* Invert everything except media */\n html {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #1a1a2e !important;\n }\n \n /* Restore images, videos, iframes, canvas */\n img, video, iframe, canvas, svg, picture, [style*=\"background-image\"] {\n filter: invert(1) hue-rotate(180deg) !important;\n }\n \n /* Preserve specific elements that should not be inverted */\n .no-dark-mode, .no-dark-mode *,\n [data-theme=\"light\"], [data-theme=\"light\"],\n .ace_editor, .ace_editor *,\n .CodeMirror, .CodeMirror *,\n .monaco-editor, .monaco-editor *,\n .markdown-body pre, .markdown-body pre *,\n .highlight, .highlight *,\n pre code, pre code * {\n filter: none !important;\n }\n \n /* Fix common UI elements */\n .modal, .popup, .dropdown-menu, .tooltip, .popover {\n filter: invert(1) hue-rotate(180deg) !important;\n background: #2d2d44 !important;\n border-color: #444 !important;\n }\n \n /* Scrollbars */\n ::-webkit-scrollbar { background: #1a1a2e !important; }\n ::-webkit-scrollbar-thumb { background: #444 !important; }\n ::-webkit-scrollbar-thumb:hover { background: #555 !important; }\n \n /* Selection */\n ::selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ::-moz-selection { background: #4ecdc4 !important; color: #1a1a2e !important; }\n ';\n }\n \n function removeDarkMode() {\n var style = document.getElementById('universal-dark-mode-style');\n if (style) style.remove();\n }\n \n // Toggle with Alt+Shift+D\n document.addEventListener('keydown', function(e) {\n if (e.altKey && e.shiftKey && e.key === 'D') {\n e.preventDefault();\n enabled = !enabled;\n if (enabled) {\n applyDarkMode();\n console.log('[Universal Dark Mode] Enabled');\n } else {\n removeDarkMode();\n console.log('[Universal Dark Mode] Disabled');\n }\n }\n });\n \n // Apply on load\n applyDarkMode();\n \n // Re-apply on dynamic content\n var observer = new MutationObserver(function(mutations) {\n if (enabled && !document.getElementById('universal-dark-mode-style')) {\n applyDarkMode();\n }\n });\n observer.observe(document.head, { childList: true });\n \n console.log('[Universal Dark Mode] Loaded - Press Alt+Shift+D to toggle');\n})();", "Universal Dark Mode");
}
} catch(__e) { console.warn('[Userscript:Universal Dark Mode]', __e); }
})();
})();

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Flags SDK
The feature flags toolkit for Next.js, SvelteKit, and Web Router.
From the creators of Next.js, the Flags SDK is a free open-source library that gives you the tools you need to use feature flags in Next.js, SvelteKit, and Web Router applications.
  • Works with any flag provider, custom setups or no flag provider at all
  • Compatible with App Router, Pages Router, and Edge Middleware
  • Built for feature flags and experimentation
See flags-sdk.dev for full docs and examples.
Fork Differences
This is a fork of the original Vercel Flags SDK with enhanced Web Router support. Key differences from the upstream project:
  • Enhanced Web Router Integration: Full-featured Web Router support with precomputation, middleware patterns, and intelligent caching
  • Framework-Specific Error Handling: Improved error messages and handling tailored for different frameworks
  • Complete Web Router Examples: Comprehensive examples demonstrating dashboard pages, marketing pages, and manual approaches
  • Consistent API Design: Implementation aligned with Next.js patterns for better developer experience
Upgrading from version 3? See the Upgrade to v4 guide.
Installation
Install the package using your package manager:
npm install flags
Setup
Create an environment variable called FLAGS_SECRET.
The FLAGS_SECRET value must have a specific length (32 random bytes encoded in base64) to work as an encryption key. Create one using node:
node -e "console.log(crypto.randomBytes(32).toString('base64url'))"
This secret is required to use the SDK. It is used to read overrides and to encrypt flag values in case they are sent to the client and should stay secret.
Usage
Create a file called flags.ts in your project and declare your first feature flag there:
// app/flags.tsximport{flag}from'flags/next';exportconstexampleFlag=flag<boolean>({key: 'example-flag',decide(){returntrue;},});
Call your feature flag in a React Server Component:
// app/page.tsximport{exampleFlag}from'./flags';exportdefaultasyncfunctionPage(){constexample=awaitexampleFlag();return<div>{example ? 'Flag is on' : 'Flag is off'}</div>;}
Feature Flags can also be called in Edge Middleware and API Routes.
Adapters
The Flags SDK has adapters for popular feature flag providers including LaunchDarkly, Optimizely, and Statsig.
Documentation
There is a lot more to the Flags SDK than shown in the example above.
See the full documentation and examples on flags-sdk.dev.
About
Flags SDK
flags-sdk.dev
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