LibJS: Fix incorrect token column values (#2401)

- initializing m_line_column to 1 in the lexer results in incorrect
  column values in tokens on the first line of input.
- not incrementing m_line_column when EOF is reached results in
  an incorrect column value on the last token.

Libraries/LibJS/Lexer.cpp

@@ -149,8 +149,12 @@ Lexer::Lexer(StringView source)
 
 void Lexer::consume()
 {
-    if (m_position >= m_source.length()) {
-        m_position = m_source.length() + 1;
+    if (m_position > m_source.length())
+        return;
+
+    if (m_position == m_source.length()) {
+        m_position++;
+        m_line_column++;
         m_current_char = EOF;
         return;
     }

Libraries/LibJS/Lexer.h

@@ -55,11 +55,11 @@ private:
     bool match(char, char, char, char) const;
 
     StringView m_source;
-    size_t m_position = 0;
+    size_t m_position { 0 };
     Token m_current_token;
-    int m_current_char = 0;
-    size_t m_line_number = 1;
-    size_t m_line_column = 1;
+    int m_current_char { 0 };
+    size_t m_line_number { 1 };
+    size_t m_line_column { 0 };
 
     struct TemplateState {
         bool in_expr;

Libraries/LibJS/MarkupGenerator.cpp

@@ -43,9 +43,7 @@ String MarkupGenerator::html_from_source(const StringView& source)
     auto lexer = Lexer(source);
     for (auto token = lexer.next(); token.type() != TokenType::Eof; token = lexer.next()) {
         auto length = token.value().length();
-        auto start = token.line_column();
-        // FIXME: Why do we need to do this magic math? This math isn't even accurate enough, code like "let x = 10" renders incorrectly.
-        start = start < 2 ? 0 : start - 2;
+        auto start = token.line_column() - 1;
 
         if (start > source_cursor) {
             builder.append(source.substring_view(source_cursor, start - source_cursor));
@@ -321,4 +319,4 @@ String MarkupGenerator::wrap_string_in_style(String source, StyleType type)
     return String::format("<span style=\"%s\">%s</span>", style_from_style_type(type).characters(), source.characters());
 }
 
-}
+}

Userland/js.cpp

@@ -528,19 +528,15 @@ int main(int argc, char** argv)
                     editor.stylize(span, styles);
             };
             editor.strip_styles();
-            StringBuilder builder;
-            builder.append(editor.line());
-            // FIXME: The lexer returns weird position information without this
-            builder.append(" ");
-            String str = builder.build();
 
             size_t open_indents = s_repl_line_level;
 
-            JS::Lexer lexer(str);
+            auto line = editor.line();
+            JS::Lexer lexer(line);
             bool indenters_starting_line = true;
             for (JS::Token token = lexer.next(); token.type() != JS::TokenType::Eof; token = lexer.next()) {
                 auto length = token.value().length();
-                auto start = token.line_column() - 2;
+                auto start = token.line_column() - 1;
                 auto end = start + length;
                 if (indenters_starting_line) {
                     if (token.type() != JS::TokenType::ParenClose && token.type() != JS::TokenType::BracketClose && token.type() != JS::TokenType::CurlyClose) {