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

vlib/strings/builder.js.v

@@ -3,32 +3,37 @@
 // that can be found in the LICENSE file.
 module strings
 
+/*
 pub struct Builder {
 mut:
 	buf []byte
 pub mut:
 	len          int
 	initial_size int = 1
-}
+}*/
+
+pub type Builder = []byte
 
 pub fn new_builder(initial_size int) Builder {
-	return Builder{[]byte{cap: initial_size}, 0, initial_size}
+	return []byte{cap: initial_size}
 }
 
 pub fn (mut b Builder) write_b(data byte) {
-	b.buf << data
+	b << data
 }
 
 pub fn (mut b Builder) write(data []byte) ?int {
 	if data.len == 0 {
 		return 0
 	}
-	b.buf << data
+	b << data
 	return data.len
 }
 
 pub fn (b &Builder) byte_at(n int) byte {
-	return b.buf[n]
+	unsafe {
+		return b[n]
+	}
 }
 
 pub fn (mut b Builder) write_string(s string) {
@@ -37,7 +42,7 @@ pub fn (mut b Builder) write_string(s string) {
 	}
 
 	for c in s {
-		b.buf << c
+		b << c
 	}
 }
 
@@ -46,14 +51,41 @@ pub fn (mut b Builder) writeln(s string) {
 		b.write_string(s)
 	}
 
-	b.buf << 10
+	b << 10
 }
 
 pub fn (mut b Builder) str() string {
 	s := ''
 
-	#for (const c of b.val.buf.arr.arr)
+	#for (const c of b.val.arr.arr)
 	#s.str += String.fromCharCode(+c)
-
+	b.trim(0)
 	return s
 }
+
+pub fn (mut b Builder) cut_last(n int) string {
+	cut_pos := b.len - n
+	x := b[cut_pos..]
+	res := x.bytestr()
+	b.trim(cut_pos)
+	return res
+}
+
+pub fn (mut b Builder) go_back_to(pos int) {
+	b.trim(pos)
+}
+
+// go_back discards the last `n` bytes from the buffer
+pub fn (mut b Builder) go_back(n int) {
+	b.trim(b.len - n)
+}
+
+// cut_to cuts the string after `pos` and returns it.
+// if `pos` is superior to builder length, returns an empty string
+// and cancel further operations
+pub fn (mut b Builder) cut_to(pos int) string {
+	if pos > b.len {
+		return ''
+	}
+	return b.cut_last(b.len - pos)
+}