js: add draft support for -skip-unused (hello_world.v works)

cmd/tools/vtest-all.v

@@ -67,10 +67,20 @@ fn get_all_commands() []Command {
 			line: '$vexe -o - examples/hello_world.v | grep "#define V_COMMIT_HASH" > /dev/null'
 			okmsg: 'V prints the generated source code to stdout with `-o -` .'
 		}
-	}
+		res << Command{
-	res << Command{
+			line: '$vexe run examples/v_script.vsh > /dev/null'
-		line: '$vexe run examples/v_script.vsh'
+			okmsg: 'V can run the .VSH script file examples/v_script.vsh'
-		okmsg: 'V can run the .VSH script file examples/v_script.vsh'
+		}
+		res << Command{
+			line: '$vexe -b js -o hw.js examples/hello_world.v'
+			okmsg: 'V compiles hello_world.v on the JS backend'
+			rmfile: 'hw.js'
+		}
+		res << Command{
+			line: '$vexe -skip-unused -b js -o hw_skip_unused.js examples/hello_world.v'
+			okmsg: 'V compiles hello_world.v on the JS backend, with -skip-unused'
+			rmfile: 'hw_skip_unused.js'
+		}
 	}
 	res << Command{
 		line: '$vexe -o vtmp cmd/v'

vlib/v/gen/js/fn.v

@@ -381,6 +381,27 @@ fn (g &JsGen) fn_gen_type(it &ast.FnDecl) FnGenType {
 	}
 }
 
+fn (mut g JsGen) is_used_by_main(node ast.FnDecl) bool {
+	mut is_used_by_main := true
+	if g.pref.skip_unused {
+		fkey := node.fkey()
+		is_used_by_main = g.table.used_fns[fkey]
+		$if trace_skip_unused_fns ? {
+			println('> is_used_by_main: $is_used_by_main | node.name: $node.name | fkey: $fkey | node.is_method: $node.is_method')
+		}
+		if !is_used_by_main {
+			$if trace_skip_unused_fns_in_js_code ? {
+				g.writeln('// trace_skip_unused_fns_in_js_code, $node.name, fkey: $fkey')
+			}
+		}
+	} else {
+		$if trace_skip_unused_fns_in_js_code ? {
+			g.writeln('// trace_skip_unused_fns_in_js_code, $node.name, fkey: $node.fkey()')
+		}
+	}
+	return is_used_by_main
+}
+
 fn (mut g JsGen) gen_fn_decl(it ast.FnDecl) {
 	res := g.fn_gen_type(it)
 	if it.language == .js {
@@ -389,6 +410,12 @@ fn (mut g JsGen) gen_fn_decl(it ast.FnDecl) {
 	if g.inside_builtin {
 		g.builtin_fns << it.name
 	}
+	if !g.is_used_by_main(it) {
+		return
+	}
+	if it.should_be_skipped {
+		return
+	}
 	cur_fn_decl := g.fn_decl
 	g.gen_method_decl(it, res)
 	g.fn_decl = cur_fn_decl