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How to check the type of a variable or object in JavaScript?

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2023-09-03 10:09:021284browse

如何在 JavaScript 中检查变量或对象的类型？

JavaScript is a loosely typed programming language, which means there are no such rules for declaring variable types. A variable can store multiple data types in a program, so it is important to understand the variable type before using it. In JavaScript, we can use the typeof operator to check the type of a variable or object. typeof Operator accepts a variable and returns its type in string format.

In addition to the typeof operator, JavaScript also provides the instanceof operator to check the type of a variable or object. instanceof The operator accepts two parameters: the object to be checked and the constructor of the type to be checked. This operator returns true if the constructor is of type object.

Use typeof operator

The

typeof operator is a unary operator that accepts a parameter and returns a string indicating the parameter type. For example, the typeof operator can be used to check the type of a variable or object.

grammar

typeof variable_name

In the above syntax, variable_name is the name of the variable whose type is to be determined.

The typeof operator can return one of the following strings -

  • "number" represents number

  • "string" means string

  • Boolean value "boolean"

  • "Undefined" means an undefined value

  • "object" means objects (including arrays and functions)

  • "symbol" for symbols (new in ECMAScript 2015)

Example

In this example, we use the typeof operator to check the type of a variable or object in JavaScript. We declared multiple variables of different types such as numbers, strings, booleans, etc. We display these variables on the web page. We used a click event handler on the button to check the type of the variable. As long as the user clicks the button, he can see all the variables and their types on the web page. The typeof operator helps determine the type of a variable or object before performing a specific operation. For example, you can use it to ensure that a variable is a number before performing arithmetic, or to ensure that a variable is a string before concatenating it with another string. 

<html>
<body>
   <h2>Checking the <i> type of a variable or object </i> in JavaScript</h2>
   <h4>The variables are as follows:</h4>
   <ul>
      <li>let num = 10</li>
      <li>let str = "Hello"</li>
      <li>let bool = true</li>
      <li>let un</li>
      <li>let n = null</li>
      <li>let arr = [1, 2, 3]</li>
      <li>let func = function () {}</li>
   </ul>
   <button onclick = "checkType()"> Check Type </button>
   <div id = "root"> </div>
   <script>
      let num = 10
      let str = 'Hello'
      let bool = true
      let un
      let n = null
      let arr = [1, 2, 3]
      let func = function () {}
      let root = document.getElementById('root')
      function checkType() { 
         root.innerHTML = '<h4>The types of variables are as follows:</h4>'
         root.innerHTML += '<ul> <li> let num = 10 is a ' + typeof num + ' </li> <li> let str = "Hello" is a ' + typeof str + ' </li> <li> let bool = true is a ' + typeof bool + ' </li> <li> let un is a ' + typeof un + ' </li> <li> let n = null is a ' + typeof n + ' </li> <li> let arr = [1, 2, 3] is a ' + typeof arr + ' </li> <li> let func = function () {} is a ' + typeof func + ' </li> </ul> '
      }
   </script>
</body>
</html>

Use instanceof operator

In JavaScript, the instanceof operator is used to determine the type of an object at runtime. It returns a Boolean result indicating whether the object is an instance of a specific class.

grammar

object_name instanceof object_constructor

In the above syntax, object_name is the name of the object whose type you want to determine.

Example

In this example, we use the instanceof operator to check the type of a variable or object in JavaScript. We declare a string type variable using the String class constructor and a custom class object "myClassObject" which is an object of "myClass" and display them on the web page. We use a click event handler on the button to check the type of object and display it on the web page. 

<html>
<body>
   <h2>Checking the <i> type of a variable or object </i> in JavaScript</h2>
   <h4>The object variables are as follows:</h4>
   <ul> 
      <li>let str = new String('Hello World!')</li>
      <li>let myClassObject = new MyClass()</li>
   </ul>
   <button onclick = "checkType()"> Check Type </button>
   <div id = "root"> </div>
   <script>
      let str = new String('Hello World!')
      class MyClass {}
      let myClassObject = new MyClass()
      let root = document.getElementById('root')
      function checkType() {
         root.innerHTML = '<h4> The types of objects using instanceof operator: </h4>'
         root.innerHTML += '<ul> <li> str is an instance of String: ' + (str instanceof String) + ' </li> <li> str is an instance of MyClass: ' + (str instanceof MyClass) + ' </li> </ul>'
         root.innerHTML += ' <ul> <li> myClassObject is an instance of String: ' + (myClassObject instanceof String) + ' </li> <li> myClassObject is an instance of MyClass: ' + (myClassObject instanceof MyClass) + ' </li> </ul>'
      }
   </script>
</body>
</html>
The

typeof and instanceof operators only sometimes return the expected results when used with certain objects. For example, the typeof operator returns "object" for arrays, even if they are an object type in JavaScript. To properly check if a value is an array, you can use the Array.isArray() method.

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JavaScript ecmascript 数据类型 String Boolean Array Object 运算符 构造函数 字符串 变量类型 字符串类型 number symbol 对象 事件 typeof 一元运算符
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