jsgen: fix string and array for loops and improve casting (#9295)

examples/.gitignore

@@ -1 +1,2 @@
 *.ppm
+*.js

vlib/builtin/js/string.v

@@ -1,6 +1,7 @@
 module builtin
 
 pub struct string {
+pub:
 	str JS.String
 	len u32
 }

vlib/v/gen/js/builtin_types.v

@@ -212,13 +212,14 @@ fn (mut g JsGen) struct_typ(s string) string {
 
 struct BuiltinPrototypeCongig {
 	typ_name 		string
-	val_name 		string = 'val'
+	val_name 		string	= 'val'
 	default_value 	string
-	constructor 	string = 'this.val = val'
-	value_of 		string = 'this.val'
-	to_string 		string = 'this.val.toString()'
-	eq				string = 'this.val === other.val'
+	constructor 	string	= 'this.val = val'
+	value_of 		string	= 'this.val'
+	to_string 		string	= 'this.val.toString()'
+	eq				string	= 'this.val === other.val'
 	extras			string
+	has_strfn		bool
 }
 
 // ugly arguments but not sure a config struct would be worth it
@@ -226,7 +227,7 @@ fn (mut g JsGen) gen_builtin_prototype(c BuiltinPrototypeCongig) {
 	g.writeln('function ${c.typ_name}(${c.val_name} = ${c.default_value}) { ${c.constructor} }')
 	g.writeln('${c.typ_name}.prototype = {')
 	g.inc_indent()
-	g.writeln('val: ${c.default_value},')
+	g.writeln('${c.val_name}: ${c.default_value},')
 	if c.extras.len > 0 {
 		g.writeln('${c.extras},')
 	}
@@ -239,7 +240,7 @@ fn (mut g JsGen) gen_builtin_prototype(c BuiltinPrototypeCongig) {
 	g.writeln('valueOf() { return ${c.value_of} },')
 	g.writeln('toString() { return ${c.to_string} },')
 	g.writeln('eq(other) { return ${c.eq} },')
-	g.writeln('str() { return new string(this.toString()) }')
+	if c.has_strfn { g.writeln('str() { return new string(this.toString()) }') }
 	g.dec_indent()
 	g.writeln('};\n')
 }
@@ -292,6 +293,7 @@ fn (mut g JsGen) gen_builtin_type_defs() {
 					value_of: 'this.str'
 					to_string: 'this.str'
 					eq: 'this.str === other.str'
+					has_strfn: false
 				})
 			}
 			'map' {

vlib/v/gen/js/js.v

@@ -690,7 +690,9 @@ fn (mut g JsGen) gen_assign_stmt(stmt ast.AssignStmt) {
 			} else {
 				g.write(' $op ')
 				// TODO: Multiple types??
-				should_cast := g.table.type_kind(stmt.left_types.first()) in js.shallow_equatables
+				should_cast := (g.table.type_kind(stmt.left_types.first()) in shallow_equatables)
+							&& (g.cast_stack.len <= 0 || stmt.left_types.first() != g.cast_stack.last())
+
 				if should_cast {
 					g.cast_stack << stmt.left_types.first()
 					if g.file.mod.name == 'builtin' {
@@ -920,36 +922,36 @@ fn (mut g JsGen) gen_for_in_stmt(it ast.ForInStmt) {
 		g.writeln('}')
 	} else if it.kind in [.array, .string] || it.cond_type.has_flag(.variadic) {
 		// `for num in nums {`
-		i := if it.key_var in ['', '_'] { g.new_tmp_var() } else { it.key_var }
-		val := if it.val_var in ['', '_'] { '' } else { it.val_var }
+		val := if it.val_var in ['', '_'] { '_' } else { it.val_var }
 		// styp := g.typ(it.val_type)
-		g.inside_loop = true
-		g.write('for (let $i = 0; $i < ')
-		g.expr(it.cond)
-		g.writeln('.len; ++$i) {')
-		g.inside_loop = false
-		if val !in ['', '_'] {
-			g.write('\tconst $val = ')
+		if it.key_var.len > 0 {
+			g.write('for (const [$it.key_var, $val] of ')
 			if it.kind == .string {
-				if g.file.mod.name == 'builtin' {
-					g.write('new ')
-				}
-				g.write('byte(')
-			}
-			g.expr(it.cond)
-			g.write(if it.kind == .array {
-				'.arr'
-			} else if it.kind == .string {
-				'.str'
+				g.write('Array.from(')
+				g.expr(it.cond)
+				g.write('.str.split(\'\').entries(), ([$it.key_var, $val]) => [$it.key_var, ')
+				if g.ns.name == 'builtin' { g.write('new ') }
+				g.write('byte($val)])')
 			} else {
-				'.val'
-			})
-			g.write('[$i]')
-			if it.kind == .string {
-				g.write(')')
+				g.expr(it.cond)
+				g.write('.entries()')
+			}
+			
+		} else {
+			g.write('for (const $val of ')
+			g.expr(it.cond)
+			if it.kind == .string {
+				g.write('.str.split(\'\')')
+			}
+			// cast characters to bytes
+			if val !in ['', '_'] && it.kind == .string {
+				g.write('.map(c => ')
+				if g.ns.name == 'builtin' { g.write('new ') }
+				g.write('byte(c))')
+				
 			}
-			g.writeln(';')
 		}
+		g.writeln(') {')
 		g.stmts(it.stmts)
 		g.writeln('}')
 	} else if it.kind == .map {
@@ -1393,7 +1395,9 @@ fn (mut g JsGen) gen_infix_expr(it ast.InfixExpr) {
 		if l_sym.kind in js.shallow_equatables && r_sym.kind in js.shallow_equatables {
 			g.expr(it.left)
 			g.write('.eq(')
+			g.cast_stack << int(l_sym.kind)
 			g.expr(it.right)
+			g.cast_stack.delete_last()
 			g.write(')')
 		} else {
 			g.write('vEq(')
@@ -1431,10 +1435,16 @@ fn (mut g JsGen) gen_infix_expr(it ast.InfixExpr) {
 		g.write(g.typ(it.right_type))
 	} else {
 		is_arithmetic := it.op in [token.Kind.plus, .minus, .mul, .div, .mod]
-		needs_cast := it.left_type != it.right_type
+		mut needs_cast := is_arithmetic && it.left_type != it.right_type
+		mut greater_typ := 0
+		if needs_cast {
+			greater_typ = g.greater_typ(it.left_type, it.right_type)
+			if g.cast_stack.len > 0 {
+				needs_cast = g.cast_stack.last() != greater_typ
+			}
+		}
 		
-		if is_arithmetic && needs_cast {
-			greater_typ := g.greater_typ(it.left_type, it.right_type)
+		if needs_cast {
 			if g.ns.name == 'builtin' {
 				g.write('new ')
 			}
@@ -1445,7 +1455,7 @@ fn (mut g JsGen) gen_infix_expr(it ast.InfixExpr) {
 		g.write(' $it.op ')
 		g.expr(it.right)
 
-		if is_arithmetic && needs_cast {
+		if needs_cast {
 			g.cast_stack.delete_last()
 			g.write(')')
 		}