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I'm confused by why this is needed.

1. If you are using babel/runtime, then that typeof expression is replaced by a call to the typeof helper, which handles polygilled symbols
2. If you are not, you should be using babel/plugin-transform-typeof-symbol anyway (so that typeof works in general), and it will make this typeof work.

I'm confused by why this is needed.

1. If you are using babel/runtime, then that typeof expression is replaced by a call to the typeof helper, which handles polygilled symbols
2. If you are not, you should be using babel/plugin-transform-typeof-symbol anyway (so that typeof works in general), and it will make this typeof work.

Thanks for your reply.
I think the point is that the current typeof res !== "object" is not transformed to _typeof(res) !== "object".
So, should I do it this way,

// packages/babel-helpers/src/helpers/toPrimitive.ts

import typeOf from "./typeof.ts";
...
if (typeOf(res) !== "object") return res;
...

or do you have any other suggestions?

I think the point is that the current typeof res !== "object" is not transformed to _typeof(res) !== "object".

But it is, both in @babel/runtime (https://unpkg.com/browse/@babel/runtime@7.25.0/helpers/esm/toPrimitive.js) and when using inline helpers (https://babeljs.io/repl#?browsers=ie%2011&build=&builtIns=false&corejs=3.21&spec=false&loose=false&code_lz=FAYwNghgzlAECCsDexawNoA8C6sC8sATMAL7DBA&debug=false&forceAllTransforms=false&modules=false&shippedProposals=false&evaluate=false&fileSize=false&timeTravel=false&sourceType=module&lineWrap=true&presets=env&prettier=true&targets=&version=7.25.3&externalPlugins=&assumptions=%7B%7D)

Interestingly, I saw another scene in @babel/helpers (https://unpkg.com/browse/@babel/helpers@7.25.0/lib/helpers/toPrimitive.js), and the mobx (https://unpkg.com/browse/mobx@6.13.1/dist/mobx.esm.js#:~:text=function-,_toPrimitive,-(t%2C%20r) really doesn't use the _typeof function.

So, is it an issue with the mobx build configuration?

Please confirm whether need to use typeOf in toPrimitive.
If not, please close this pr.

I investigated this a bit. The reason seems (seems because I cannot find Mobx's Babel config) that Mobx is not transpiled to support IE11, and thus the injected _typeof helper does not support the symbol polyfill for IE11.

There are two solutions:

• Mobx starts transpiling their code targeting IE11
• You (user of Mobx) have to transpile Mobx's code with Babel from your node_modules, targeting IE11.