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Feature: Interpret Magic Comments for preset-env to add polyfills #9072

axten opened this Issue Nov 23, 2018 · 3 comments


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axten commented Nov 23, 2018

Feature Request

Is your feature request related to a problem? Please describe.
It would be great if babel/preset-env could interpret magic comments in files in order to add builtIn polyfills.

Describe the solution you'd like
An example: Assume I have a config like this

presets: [
    ['@babel/preset-env', {
        modules: false,
        useBuiltIns: 'usage',
        targets: {
            "chrome": "58",
            "ie": "11"

and a library like Choices.js. It is shipped as ES5 node module but it uses some modern features like Array.find (see complete list here). Normally, node_modules are excluded from babel so preset-env has no chance to detect these used features and include the polyfills. So now my idea comes in....

Wouldn't it be nice to have something like that to make it work:

/* babel-used-buildins array.find, Object.assign */
import Choices from 'choices.js';

const input = new Choices(element, options);

Describe alternatives you've considered
Sure, it may exist a way to achieve this with config values but for me, thats a kind of polluting the config.

Teachability, Documentation, Adoption, Migration Strategy
later :)


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babel-bot commented Nov 23, 2018

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nicolo-ribaudo commented Nov 24, 2018

Note that I think that you could already do something like this:

useBuiltIns: [].find, Object.assign;
import Choices from 'choices.js';

const input = new Choices(element, options);

EDIT: It works 🙂

import "core-js/modules/es6.object.assign";
import "core-js/modules/es6.array.find";

useBuiltIns: [].find, Object.assign;

import Choices from 'choices.js';
var input = new Choices(element, options);

A minifier can then easily detect that useBuiltIns: [].find, Object.assign; doesn't have side effects and remove it.
If you prefer, you can use Array.prototype.find instead of [].find


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axten commented Nov 24, 2018

Ok, thats definitely an alternative I haven't considered yet :)
For the record: I think that a magic comment would be a bit nicer for co-workers when reading the code later on (in terms of maintainability)...
Therefore I will leave this open for a while... maybe someone like to land a PR for that :)
thanks @nicolo-ribaudo !

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