--- title: Deep dive into the new Firebase JS SDK design | daily.dev description: Firebase's JS SDK version 9 moves from a namespace-and-service pattern to a modular, function-based API to enable tree shaking and smaller bundle sizes. The... canonical: https://daily.dev/posts/deep-dive-into-the-new-firebase-js-sdk-design-eyn5um4wo twitter:card: summary_large_image twitter:site: @dailydotdev og:type: website og:site_name: daily.dev og:title: Deep dive into the new Firebase JS SDK design | daily.dev og:description: Firebase's JS SDK version 9 moves from a namespace-and-service pattern to a modular, function-based API to enable tree shaking and smaller bundle sizes. The... og:url: https://daily.dev/posts/deep-dive-into-the-new-firebase-js-sdk-design-eyn5um4wo og:image: https://api.daily.dev/og/posts/eYN5UM4Wo.png og:image:alt: Deep dive into the new Firebase JS SDK design og:image:width: 1200 og:image:height: 630 og:locale: en --- > ## Documentation Index > Fetch the complete documentation index at: https://daily.dev/llms.txt > Use this file to discover all available pages before exploring further. # Deep dive into the new Firebase JS SDK design **[Firebase](https://daily.dev/sources/firebase-blog)** · 10 min read · 0 upvotes · 0 comments ## Summary Firebase's JS SDK version 9 moves from a namespace-and-service pattern to a modular, function-based API to enable tree shaking and smaller bundle sizes. The new SDK replaces chained namespace methods (e.g., firebase.auth()) with standalone functions (e.g., getAuth(firebaseApp)), reducing unused code in bundles. A compatibility library is provided so developers can incrementally migrate from v8 to v9 by swapping import paths and mixing old and new APIs during the transition. The library was in beta at the time of writing. ## Full article daily.dev links to this article rather than hosting it. Read it at the original source: ## Questions this post answers ### How do I migrate my code from Firebase JS SDK v8 to the new modular v9 syntax for auth? Replace namespaced calls like firebase.auth() and auth.onAuthStateChanged() with modular functions: import getAuth and onAuthStateChanged from 'firebase/auth', call const auth = getAuth(firebaseApp), then onAuthStateChanged(auth, callback). Use the firebase/compat/app and firebase/compat/auth import paths during a gradual migration to run old and new APIs side by side. _Track breaking SDK migrations like this one on daily.dev before they hit your production build._ ### Why did Firebase move from a namespace-based API to a modular function-based API in its JavaScript SDK? The namespace pattern (firebase.auth() chaining) prevented tree shaking because bundlers could not detect which methods on a chain were unused, forcing inclusion of all nine Firebase Auth sign-in methods even if unused. The new modular v9 SDK exports individual functions like getAuth() and initializeApp() instead of a catch-all namespace, letting tools like Rollup and Webpack eliminate unused code and shrink bundle size. _Developers choosing between SDK architectures follow API redesigns like this via daily.dev._ ### What is the Firebase compatibility library used for when upgrading to the modular v9 SDK? The compatibility library lets developers keep using the old v8-style namespaced API while running on the new v9 SDK internally, by importing from firebase/compat/app and firebase/compat/auth instead of firebase/app and firebase/auth. This allows old and new APIs to coexist so teams can upgrade incrementally without rewriting everything at once. _daily.dev helps developers planning gradual SDK upgrades stay on top of migration guides._ --- Tags: [#webdev](https://daily.dev/tags/webdev), [#javascript](https://daily.dev/tags/javascript), [#firebase](https://daily.dev/tags/firebase) [View this post on daily.dev](https://daily.dev/posts/deep-dive-into-the-new-firebase-js-sdk-design-eyn5um4wo) ```json {"@context":"https://schema.org","@graph":[{"@type":"Organization","@id":"https://daily.dev/#organization","name":"daily.dev","url":"https://daily.dev","logo":{"@type":"ImageObject","url":"https://daily.dev/apple-touch-icon.png","width":180,"height":180},"sameAs":["https://twitter.com/dailydotdev","https://github.com/dailydotdev","https://www.linkedin.com/company/daily-dev-ltd"]},{"@type":"WebSite","@id":"https://daily.dev/#website","url":"https://daily.dev","name":"daily.dev","publisher":{"@id":"https://daily.dev/#organization"},"potentialAction":{"@type":"SearchAction","target":{"@type":"EntryPoint","urlTemplate":"https://daily.dev/search?q={search_term_string}"},"query-input":"required name=search_term_string"}}]} {"@context":"https://schema.org","@type":"TechArticle","headline":"Deep dive into the new Firebase JS SDK design","url":"https://daily.dev/posts/deep-dive-into-the-new-firebase-js-sdk-design-eyn5um4wo","mainEntityOfPage":{"@type":"WebPage","@id":"https://daily.dev/posts/deep-dive-into-the-new-firebase-js-sdk-design-eyn5um4wo"},"datePublished":"2026-08-23T12:19:31.953Z","dateModified":"2026-08-23T12:31:33.535Z","description":"Firebase's JS SDK version 9 moves from a namespace-and-service pattern to a modular, function-based API to enable tree shaking and smaller bundle sizes. 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