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JavaScript in 运算符
tryjsref_oper_in_objects
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<!DOCTYPE html> <html> <body> <h1>JavaScript Operators</h1> <h2>The in Operator</h2> <p>The in operator returns true if a property is in an object.</p> <p id="demo"></p> <script> const person = {firstName:"John", lastName:"Doe", age:50}; document.getElementById("demo").innerHTML = ("firstName" in person) + "<br>" + ("age" in person) + "<br>" + ("address" in person); </script> </body> </html>