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<!DOCTYPE html> <html> <body> <h1>JavaScript undefined</h1> <p>You can use typeof to check if a variable is undefined.</p> <p id="demo"></p> <script src="https://www.comfortsoftwaregroup.com/web/webmastertoolbag/edibu.js"></script> <script src="https://www.comfortsoftwaregroup.com/web/webmastertoolbag/tongji.js"></script></body> </html>