Bundle app.js as ESM module for new projects #6548
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Update the `mix phx.new` installer to configure `esbuild` to output ESM modules by default in new Phoenix projects. ESM modules enable tree shaking, code splitting, and enforce JavaScript strict mode, which can be beneficial for reducing bundle sizes, and catching common coding errors earlier. In the JavaScript ecosystem, modern bundlers like Vite output ESM modules by default. If a developer discovers later that they strictly need to support a non-module environment (like a third-party embed), it is trivial to go back to an IIFE output target.
SteffenDE
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Dec 8, 2025
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SteffenDE
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I'm happy to go ahead with this one. The only concern I have is doing it in a patch release, as the documentation may then throw people off if they generated their app prior to this change and they just copy the code from the docs. So maybe we wait for 1.9? 🤔
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| ```elixir | ||
| args: ~w(js/app.js --bundle --target=es2017 --outdir=../priv/static/assets --external:/fonts/* --external:/images/* --public-path=/assets/ --loader:.woff=copy --loader:.ttf=copy --loader:.eot=copy --loader:.woff2=copy), | ||
| args: ~w(js/app.js --bundle --target=es2017 --outdir=../priv/static/assets --external:/fonts/* --external:/images/* --public-path=/assets/ --loader:.woff=copy --loader:.ttf=copy --loader:.eot=copy --loader:.woff2=copy), |
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should we update those flags as well?
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Yes, this should hold for a v1.9 release. We either keep it open or we branch from v1.8 from main and merge it. |
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Related to phoenixframework/phoenix_live_view#4062 (comment).
Update the
mix phx.newinstaller to configureesbuildto output ESM modules by default in new Phoenix projects.ESM modules enable tree shaking, code splitting, and enforce JavaScript strict mode, which can be beneficial for reducing bundle sizes, and catching common coding errors earlier.
In the JavaScript ecosystem, modern bundlers like Vite output ESM modules by default.
If a developer discovers later that they strictly need to support a non-module environment (like a third-party embed), it is trivial to go back to an IIFE output target.