How does Symbol.hasInstance + instanceof work (or does it even work?) in IE11 and below?

[trusktr]

I'm looking at this output in the repl.

From what I can tell, it requires a Symbol.hasInstance polyfill.

However, the instanceof check isn't being transpiled into anything.

So, in an older browser like IE 10, how can this possibly work if the native instanceof operator will not know to look for Symbol.hasInstance on a constructor?

In otherwords, doesn't

f instanceof Foo;

need to be transpiled to something like

__instanceOf(f, Foo);

where __instanceOf is a helper that can use Symbol.hasInstance (polyfilled or not)?

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[jridgewell]

Does babel-polyfill not handle this?

[zloirock]

@jridgewell core-js provide only Symbol.hasInstance and Function.prototype.@@hasInstance, but can't override instanceof operator logic.

[nicolo-ribaudo]

This is handled by the babel-plugin-transform-es2015-instanceof plugin. It needs to be added to babel-preset-es2015 and babel-preset-env

[ajaymathur]

Hi, Can I take this up please?

[loganfsmyth]

This is an interesting one. I wonder if it was left out on purpose? I could see this being a performance issue, but I honestly don't know. Seems like we'd need to dig into the git history?

[ajaymathur]

@loganfsmyth I did a search on git log and found following commit:

commit a9ac3b0
Author: Sebastian McKenzie sebmck@gmail.com
Date: Tue Nov 3 19:22:51 2015 +0000

move instanceof functionality to separate plugin - fixes #2745

direct link to issue #2745

[jridgewell]

Likely just an oversight then, I don't see it being any more of a penalty than typeof.

[matiasfha]

So this is basically re-add the functionality to preset-env preset-es2015 as mentioned in #6210 (comment)

[jridgewell]

Yup.

[ajaymathur]

Cool, so can I please take this up ?

[nicolo-ribaudo]

@ajaymathur Yes 👍