undefined as default parameter is preserved since 5.16.6

[ericfortis]

terser input

function foo(a = undefined) { return a; }
alert(foo())

terser output or error (5.16.6+)

alert(function(n=undefined){return n}());

Expected result (5.15.5)

alert(function(n){return n}());

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Why do I write signatures like that?
In WebStorm, there's a "Signature Mismatch" inspection but it doesn't handle optional parameters. Therefore, in those cases I add undefined as default value.

[fabiosantoscode]

I'm noticing more than a few optimization opportunities here (all that should be just alert(void 0) but the default parameter is being preserved because removing it changes the function's .length property.

I guess it can be done if we know about all the function's calls.

[fabiosantoscode]

I'm pushing a partial fix (it's really fast and easy to detect a function is an IIFE). For your case (which I'm sure is not just IIFE) you can try disabling the keep_fargs option, which makes functions' lengths not important, and therefore = undefined considered safe to remove.