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How to use 'in' operator in JavaScript?

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2023-09-12 17:13:09869browse

如何在 JavaScript 中使用“in”运算符？

In this article, we will explore the “in” operator and how to use it in JavaScript. The in operator is a built-in operator in JavaScript that is used to check whether a specific property exists in an object. Returns true if the property exists, false otherwise.

Syntax

prop in object

Parameters

This function accepts the following parameters as mentioned below-

  • prop - This parameter holds a string or symbol representing the property name or array index.

  • object - will check if the object contains prop .

Return value - if The specified property is found in the object, this method returns true or false.

Example 1

In the following example, we will use the 'inin' operator in JavaScript to find whether a property exists.

# index .html

<html>
<head>
   <title>IN operator</title>
</head>
<body>
   <h1 style="color: red;">
      Welcome To Tutorials Point
   </h1>
   <script>
      // Illustration of in operator
      const array = [&#39;key&#39;, &#39;value&#39;, &#39;title&#39;, &#39;TutorialsPoint&#39;]
      
      // Output of the indexed number
      console.log(0 in array) //true
      console.log(2 in array) //true
      console.log(5 in array) //false
      
      // Output of the Value
      // you must specify the index number, not the value at that index
      console.log(&#39;key&#39; in array) //false
      console.log(&#39;TutorialsPoint&#39; in array) // false
      
      // output of the Array property
      console.log(&#39;length&#39; in array)
   </script>
</body>
</html>

Output

The above program will produce the following output in the console. 

true
true
false
false
false
true

Example 2

In the following example, we demonstrate the in operator.

# index.html p>

<html>
<head>
   <title>IN operator</title>
</head>
<body>
   <h1 style="color: red;">
      Welcome To Tutorials Point
   </h1>
   <script>
      // Illustration of in operator
      const student = { name: &#39;Bill&#39;, class: &#39;IX&#39;, subjects: &#39;PCM&#39;, age: &#39;16&#39; };
      console.log(&#39;name&#39; in student);
      delete student.name;
      console.log(&#39;name&#39; in student);
      if (&#39;name&#39; in student === false) {
         student.name = &#39;Steve&#39;;
      }
      console.log(student.name);
   </script>
</body>
</html>

Output

The above program will produce the following results in the console. 

true
false
Steve

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