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<!DOCTYPE html> <html lang="en"> <head> <meta charset="UTF-8"> <title>JavaScript undefined</title> </head> <body> <pre> <!-- Use pre element for work document.writeln() method --> <script type="text/javascript"> var str; // Identify as undefined value. document.writeln("str == null : ", str == null); document.writeln("undefined == null : ", undefined == null); var bool = Boolean(str); document.writeln("bool : ", bool); // Boolean context Returns false </script> </pre> </body> </html>