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Feature Flags in JavaScript: How to Enable Features Without Risk or Deployment
\n\nImagine: you've written a cool new feature — smart search with autocomplete. The code is ready, tests pass, but you're afraid to roll it out to all users at once. What if there's a bug? What if someone doesn't like it? In the past, you'd stay up all night deploying and then roll back versions. Today, there's an elegant solution — feature flags (or feature toggles).
\n\nFeature flags are flags in code that allow you to enable or disable functionality without redeploying. Simply change a value in a config or admin panel — and the feature comes to life or disappears. In this article, we'll break down how to use them in JavaScript, show real code examples, and provide a checklist for implementation.
\n\nWhat are feature flags and why are they needed in JS?
\n\nFeature flags (also known as feature toggles) are simple conditional constructs that control whether to show a new feature to a user or not. They work at the code level: instead of deleting old code, you wrap the new code in a flag check.
\n\nWhy is this needed in JavaScript projects?
\n- \n
- Canary release — roll out a feature to 5% of users, monitor for errors, then expand. \n
- A/B testing — show different interface versions to different groups and compare metrics. \n
- Instant kill switch — if a feature breaks production, you simply disable the flag without deploying. \n
- Trunk-based development — you can merge incomplete code hidden behind a flag. \n
Simplest feature flag in JavaScript: example
\n\nLet's start with the most basic option — a flag in a config. Suppose we have a search function, and we want to enable autocomplete only for testers.
\n\n// config.js\nconst featureFlags = {\n enableAutocomplete: false, // disabled by default\n enableDarkMode: true\n};\n\n// search.js\nfunction initSearch() {\n if (featureFlags.enableAutocomplete) {\n console.log('Autocomplete enabled');\n loadAutocompleteModule();\n } else {\n console.log('Autocomplete disabled');\n loadBasicSearch();\n }\n}\nChange enableAutocomplete to true in the config — and the feature is enabled. Simple? Yes. But this is just the beginning. For production, you need more — dynamic flags, API-based management, A/B tests.
Advanced approach: libraries for feature flags
\n\nYou can write your own custom flag solutions, but it's better to use proven libraries. Here are the top 3 for JavaScript:
\n\n- \n
- LaunchDarkly — the corporate standard. Has SDKs for React, Vue, Node.js. Supports A/B testing, segmentation, percentage-based rollout. \n
- Flagsmith — an open-source alternative. Supports self-hosted and cloud. Works with any framework. \n
- Unleash — a lightweight open-source library. Great for smaller projects. \n
Example with Unleash:
\n\n// Installation: npm install unleash-client\nconst { initialize } = require('unleash-client');\n\nconst unleash = initialize({\n url: 'https://your-unleash-instance.com/api',\n appName: 'my-js-app',\n environment: 'production',\n customHeaders: { Authorization: 'API-TOKEN' }\n});\n\n// Check flag\nconst isEnabled = unleash.isEnabled('new-checkout-flow');\nif (isEnabled) {\n renderNewCheckout();\n} else {\n renderOldCheckout();\n}\nSuch libraries sync with the server in real time. If you change a flag on the dashboard, the client picks it up within a few seconds (usually 30-60 seconds, configurable).
\n\nFeature flags in React: Hooks and context
\n\nIn React applications, it's convenient to use context and custom hooks. Here's an example architecture:
\n\n
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