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JavaScript in 运算符
tryjsref_oper_in_predefined
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<!DOCTYPE html> <html> <body> <h1>JavaScript Operators</h1> <h2>The in Operator</h2> <p id="demo"></p> <script> document.getElementById("demo").innerHTML = ("PI" in Math) + "<br>" + ("NaN" in Number) + "<br>" + ("length" in String); </script> </body> </html>