From 8bd5cf064bb488e9f92986ab20ac30bd468ccf2e Mon Sep 17 00:00:00 2001
From: Alexander Medvednikov <alexander@medvednikov.com>
Date: Tue, 21 Apr 2026 16:52:39 +0300
Subject: [PATCH] jsgen: fix js-backend cannot compile life.v (fixes #20667)

---
 vlib/builtin/js/builtin.js.v | 29 +++++++++++++++++++++++++++++
 vlib/v/gen/js/jsgen_test.v   | 12 ++++++++++++
 2 files changed, 41 insertions(+)

diff --git a/vlib/builtin/js/builtin.js.v b/vlib/builtin/js/builtin.js.v
index 7c24a1913..c24330f4d 100644
--- a/vlib/builtin/js/builtin.js.v
+++ b/vlib/builtin/js/builtin.js.v
@@ -2,6 +2,10 @@ module builtin
 
 // used to generate JS throw statements.
 
+$if js_node {
+	#var $fs = require('fs');
+}
+
 @[noreturn]
 pub fn js_throw(s any) {
 	#throw s
@@ -57,6 +61,31 @@ pub fn eprint(s string) {
 	}
 }
 
+// input_character gives back a single character, read from the standard input.
+// It returns -1 on error (when the input is finished (EOF), on a broken pipe etc).
+pub fn input_character() int {
+	$if js_node {
+		mut ch := -1
+		#try {
+		#const read_buffer = Buffer.alloc(1)
+		#const nbytes = $fs.readSync(0, read_buffer, 0, 1, null)
+		#if (nbytes > 0) { ch.val = read_buffer[0] }
+		#} catch (e) {}
+
+		return ch
+	} $else {
+		return -1
+	}
+}
+
+// print_character writes the single character `ch` to the standard output.
+// It returns the written character value, or panics if the active JS runtime
+// does not support stdout writes.
+pub fn print_character(ch u8) int {
+	print(ch.ascii_str())
+	return ch
+}
+
 // Exits the process in node, and halts execution in the browser
 // because `process.exit` is undefined. Workaround for not having
 // a 'real' way to exit in the browser.
diff --git a/vlib/v/gen/js/jsgen_test.v b/vlib/v/gen/js/jsgen_test.v
index 593acb871..91e90bfbf 100644
--- a/vlib/v/gen/js/jsgen_test.v
+++ b/vlib/v/gen/js/jsgen_test.v
@@ -76,6 +76,18 @@ fn test_issue_11379_js_browser_builtin_print_call_does_not_panic() {
 	assert os.exists(output)
 }
 
+fn test_issue_20667_js_can_compile_game_of_life_example() {
+	vexe := os.getenv('VEXE')
+	os.chdir(os.dir(vexe)) or {}
+	os.mkdir_all(output_dir) or { panic(err) }
+	program := os.join_path('examples', 'game_of_life', 'life.v')
+	output := os.join_path(output_dir, 'game_of_life.js')
+	res :=
+		os.execute('${os.quoted_path(vexe)} -prod -b js -w -o ${os.quoted_path(output)} ${os.quoted_path(program)}')
+	assert res.exit_code == 0, res.output
+	assert os.exists(output)
+}
+
 fn find_test_files() []string {
 	files := os.ls(test_dir) or { panic(err) }
 	// The life example never exits, so tests would hang with it, skip
-- 
2.39.5