jsgen: maps created on the fly do not have a valid size

[gchumillas]

Describe the bug

Maps created on the fly do not have a valid size.

Reproduction Steps

Create a small main.v app:

fn main() {
	mut m := {'one': 1, 'two': 2}
	assert m.len == 2

	/* However, this works!
	
	mut m := map[string]int{}
	m['one'] = 1
	m['two'] = 2
	assert m.len == 2 */
}

Compile and execute it:

v -b js main.v
node main.js

Output:

FAIL: fn main.main(): assert m.len == 2

Expected Behavior

You should not complain.

Current Behavior

It complains.

Possible Solution

No response

Additional Information/Context

No response

V version

V 0.4.10 0d00eb7

Environment details (OS name and version, etc.)

V full version	V 0.4.10 52c7130.0d00eb7
OS	macos, macOS, 15.5, 24F74
Processor	8 cpus, 64bit, little endian, Apple M1 Pro
Memory	0.53GB/16GB
V executable	/Users/gonzalo/Projects/Personal/vlang/v
V last modified time	2025-05-30 11:52:55
V home dir	OK, value: /Users/gonzalo/Projects/Personal/vlang
VMODULES	OK, value: /Users/gonzalo/.vmodules
VTMP	OK, value: /tmp/v_501
Current working dir	OK, value: /Users/gonzalo/Projects/Personal/vlang
Git version	git version 2.39.1
V git status	weekly.2025.22-20-g0d00eb71
.git/config present	true
cc version	Apple clang version 17.0.0 (clang-1700.0.13.5)
gcc version	Apple clang version 17.0.0 (clang-1700.0.13.5)
clang version	Apple clang version 17.0.0 (clang-1700.0.13.5)
tcc version	tcc version 0.9.28rc 2024-02-05 HEAD@105d70f7 (AArch64 Darwin)
tcc git status	thirdparty-macos-arm64 e447816c
emcc version	N/A
glibc version	N/A

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[huly-for-github]

Connected to Huly®: V_0.6-22977