Confused About UseBuiltIns Option Of @babel/preset-env (using Browserslist Integration)


Answer :


1) Do I need to use that useBuiltIns: "entry" option?




Yes, if you want to include polyfills based on your target environment.



TL;DR



There're basically 3 options for useBuiltIns:



"entry": when using this option, @babel/preset-env replaces direct imports of core-js to imports of only the specific modules required for a target environment.



That means you need to add



import "core-js/stable";
import "regenerator-runtime/runtime";


to your entry point and these lines will be replaced by only required polyfills. When targeting chrome 72, it will be transformed by @babel/preset-env to



import "core-js/modules/es.array.unscopables.flat";
import "core-js/modules/es.array.unscopables.flat-map";
import "core-js/modules/es.object.from-entries";
import "core-js/modules/web.immediate";




"usage": in this case polyfills will be added automatically when the usage of some feature is unsupported in target environment. So:



const set = new Set([1, 2, 3]);
[1, 2, 3].includes(2);


in browsers like ie11 will be replaced with 



import "core-js/modules/es.array.includes";
import "core-js/modules/es.array.iterator";
import "core-js/modules/es.object.to-string";
import "core-js/modules/es.set";

const set = new Set([1, 2, 3]);
[1, 2, 3].includes(2);


In case target browser is latest chrome, no transformations will apply.



That's personally my chosen weapon as there's no need to include anything (core-js or regenerator) in source code as only required polyfills will be added automatically based on target environment set in browserlist.





false: that's the default value when no polyfills are added automatically.






2) Do I need to install @babel/polyfill package and start my
vendors.js with require("@babel/polyfill"); ?




Yes for environment prior to babel v7.4 and core-js v3.



TL;DR



No. Starting from babel v7.4 and core-js v3 (which is used for polyfilling under the hood) @babel/preset-env will add the polyfills only when it know which of them required and in the recommended order.



Moreover @babel/polyfill is considered as deprecated in favor of separate core-js and regenerator-runtime inclusions.



So using of useBuiltIns with options other than false should solve the issue.



Don't forget to add core-js as a dependency to your project and set its version in @babel/preset-env under corejs property.






3) What if I omit both?




As @PlayMa256 already answered, there will be no polyfills.





More detailed and whole info con be found at core-js creator's page



Also please feel free to play with babel sandbox




1) Do I need to use that useBuiltIns: "entry" option?




Yes, according to babel docs:



"This option enables a new plugin that replaces the statement import "@babel/polyfill" or require("@babel/polyfill") with individual requires for @babel/polyfill based on environment" - Basically, includes all needed polyfills (when you have @babel/polyfill installed when needed).




2) Do I need to install @babel/polyfill package and start my vendors.js with require("@babel/polyfill"); ?




You do need to install @babel/polyfill, it does not come by default on babel. You have to include that on your entrypoint or add an import at the top of your entrypoint.




3) What if I omit both?




You won't have polyfills.



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