JS Backend: sum type doesn't work well with array types

[gchumillas]

Describe the bug

When compiling this code using v -b js_browser .:

type SumType = string | []string

fn main() {
	x := SumType('aaa')
	match x {
		string { println('string') }
		[]string { println('[]string') }
	}
}

throws the following error:

Uncaught SyntaxError: Unexpected identifier 'string' (at main.js:7185:27)

/** @type {main__SumType} */
const x = new string("aaa");
if (x instanceof string) {
	builtin__println (new string("string"));
}
else if (x instanceof []string) { // <!-- HERE!
	builtin__println (new string("[]string"));
}

Reproduction Steps

As above.

Expected Behavior

The sum types should work well with array types.

Current Behavior

It throws a JavaScript error.

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.4.10 d559a62

Environment details (OS name and version, etc.)

V full version	V 0.4.10 30ececc.d559a62
OS	macos, macOS, 15.3.2, 24D81
Processor	8 cpus, 64bit, little endian, Apple M1 Pro
Memory	2.16GB/16GB
V executable	/Users/gonzalo/v/v
V last modified time	2025-04-13 10:51:40
V home dir	OK, value: /Users/gonzalo/v
VMODULES	OK, value: /Users/gonzalo/.vmodules
VTMP	OK, value: /tmp/v_501
Current working dir	OK, value: /Users/gonzalo/Projects/Personal/v/rx
Git version	git version 2.39.1
V git status	weekly.2025.14-39-gd559a62c (12 commit(s) behind V master)
.git/config present	true
cc version	Apple clang version 17.0.0 (clang-1700.0.13.3)
gcc version	Apple clang version 17.0.0 (clang-1700.0.13.3)
clang version	Apple clang version 17.0.0 (clang-1700.0.13.3)
tcc version	tcc version 0.9.28rc 2024-02-05 HEAD@105d70f7 (AArch64 Darwin)
tcc git status	thirdparty-macos-arm64 713692d4
emcc version	N/A
glibc version	N/A

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[huly-for-github]

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