​x
 
<!DOCTYPE html>
<html>
<body>
​
<p>如果指定的属性在指定的对象中，则 in 运算符返回 true。</p>
​
<p id="demo"></p>
​
<script>
// 数组
var cars = ["Audi", "Volvo", "BMW"];
// 对象
var person = {firstName:"Bill", lastName:"Gates", age:19};
​
document.getElementById("demo").innerHTML =
  ("Audi" 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>
​