Tryit Editor v2.9

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<!DOCTYPE html>
<html>
<body>

<p>The in operator returns true if the specified property is in the specified object.</p>

<p id="demo"></p>

<script>
// Arrays
var cars = ["Saab", "Volvo", "BMW"];
// Objects
var person = {firstName:"John", lastName:"Doe", age:50};

document.getElementById("demo").innerHTML =
    ("Saab" in cars) + "<br>" + 
    (0 in cars) + "<br>" +
    (1 in cars) + "<br>" +
    (4 in cars) + "<br>" +
    ("length" in cars) + "<br>" +

    ("firstName" in person) + "<br>" +
    ("age" in person) + "<br>" +

    // Predefined objects
    ("PI" in Math) + "<br>" +
    ("NaN" in Number) + "<br>" +
    ("length" in String);
</script>

</body>
</html>