js: incorrect behavior when minifying optional chaining with bracket notation (a?.["b"])

[ksw2000]

Description

When using the minifier to process JavaScript code that includes optional chaining with bracket notation, such as a?.["b"], the output is incorrect. This issue appears to be the root cause of #744 , where similar behavior was observed. The specific problem lies in how the minifier transforms a?.["b"] into a?..b, which is invalid JavaScript.

Reproduce

In js/js_test.go

func TestOptionalChainingWithBracket (t *testing.T) {
	m := minify.New()
	o := Minifier{KeepVarNames: true, useAlphabetVarNames: true}
	r := bytes.NewBufferString(`
		let a = {
			"b": "foo"
		};
		console.log(a.b);
		console.log(a?.b);
		console.log(a["b"]);
		console.log(a?.["b"]);    // Unexpected Behavior Here!!
	`)
	w := &bytes.Buffer{}
	o.Minify(m, w, r, nil)
	fmt.Println(w)
}

go test --run TestOptionalChainingWithBracket  -v

output:

let a={b:"foo"};console.log(a.b),console.log(a?.b),console.log(a.b),console.log(a?..b)

The last line of the output console.log(a?..b) is incorrect. The minifier improperly handles the bracket notation in optional chaining, resulting in invalid JavaScript syntax.

Expected Behavior

The minified code should correctly preserve the bracket notation in optional chaining. The expected output should be:

let a={b:"foo"};console.log(a.b),console.log(a?.b),console.log(a.b),console.log(a?.["b"])

---

[tdewolff]

Fixed in v2.21.1, thank you!