js: fix string.bytes codegen, readline, add tests for strings (#12060)

vlib/strings/builder_test.js.v

@@ -0,0 +1,94 @@
+import strings
+
+type MyInt = int
+
+const maxn = 100000
+
+fn test_sb() {
+	mut sb := strings.new_builder(100)
+	sb.write_string('hi')
+	sb.write_string('!')
+	sb.write_string('hello')
+	assert sb.len == 8
+	sb_end := sb.str()
+	assert sb_end == 'hi!hello'
+	assert sb.len == 0
+	///
+	sb = strings.new_builder(10)
+	sb.write_string('a')
+	sb.write_string('b')
+	assert sb.len == 2
+	assert sb.str() == 'ab'
+	// Test interpolation optimization
+	sb = strings.new_builder(10)
+	x := 10
+	y := MyInt(20)
+	sb.writeln('x = $x y = $y')
+	res := sb.str()
+	assert res[res.len - 1] == `\n`
+	println('"$res"')
+	assert res.trim_space() == 'x = 10 y = 20'
+	//
+	sb = strings.new_builder(10)
+	sb.write_string('x = $x y = $y')
+	assert sb.str() == 'x = 10 y = 20'
+	//$if !windows {
+	sb = strings.new_builder(10)
+	sb.write_string('123456')
+	last_2 := sb.cut_last(2)
+	assert last_2 == '56'
+	final_sb := sb.str()
+	assert final_sb == '1234'
+	//}
+}
+
+fn test_big_sb() {
+	mut sb := strings.new_builder(100)
+	mut sb2 := strings.new_builder(10000)
+	for i in 0 .. maxn {
+		sb.writeln(i.str())
+		sb2.write_string('+')
+	}
+	s := sb.str()
+	lines := s.split_into_lines()
+	assert lines.len == maxn
+	assert lines[0] == '0'
+	assert lines[1] == '1'
+	assert lines[777] == '777'
+	assert lines[98765] == '98765'
+	println(sb2.len)
+	assert sb2.len == maxn
+}
+
+fn test_byte_write() {
+	mut sb := strings.new_builder(100)
+	temp_str := 'byte testing'
+	mut count := 0
+	for word in temp_str {
+		sb.write_b(word)
+		count++
+		assert count == sb.len
+	}
+	sb_final := sb.str()
+	assert sb_final == temp_str
+}
+
+fn test_strings_builder_reuse() {
+	mut sb := strings.new_builder(256)
+	sb.write_string('world')
+	assert sb.str() == 'world'
+	sb.write_string('hello')
+	assert sb.str() == 'hello'
+}
+
+fn test_cut_to() {
+	mut sb := strings.new_builder(16)
+	sb.write_string('hello')
+	assert sb.cut_to(3) == 'lo'
+	assert sb.len == 3
+	assert sb.cut_to(3) == ''
+	assert sb.len == 3
+	assert sb.cut_to(0) == 'hel'
+	assert sb.cut_to(32) == ''
+	assert sb.len == 0
+}