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diff --git a/Libraries/LibJS/Lexer.h b/Libraries/LibJS/Lexer.h
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+/*
+ * Copyright (c) 2020, Stephan Unverwerth <s.unverwerth@gmx.de>
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright notice, this
+ *    list of conditions and the following disclaimer.
+ *
+ * 2. Redistributions in binary form must reproduce the above copyright notice,
+ *    this list of conditions and the following disclaimer in the documentation
+ *    and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#pragma once
+
+#include "Token.h"
+
+#include <AK/HashMap.h>
+#include <AK/String.h>
+#include <AK/StringView.h>
+
+namespace JS {
+
+class Lexer {
+public:
+    explicit Lexer(StringView source);
+    Token next();
+
+private:
+    void consume();
+    bool is_eof() const;
+    bool is_identifier_start() const;
+    bool is_identifier_middle() const;
+    bool is_line_comment_start() const;
+    bool is_block_comment_start() const;
+    bool is_block_comment_end() const;
+
+    StringView m_source;
+    size_t m_position = 0;
+    Token m_current_token;
+    int m_current_char;
+
+    static HashMap<String, TokenType> s_keywords;
+    static HashMap<String, TokenType> s_two_char_tokens;
+    static HashMap<char, TokenType> s_single_char_tokens;
+};
+
+}