JavaScript - For...in Loop

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The for...in Loop

The for...in loop in JavaScript is used to loop through an object's properties. The JavaScript for...in loop is a variant of the for loop. The for loop can't be used to traverse through the object properties. So, the for...in loop is introduced to traverse through all object properties.

As we have not discussed Objects yet, you may not feel comfortable with this loop. But once you understand how objects behave in JavaScript, you will find this loop very useful.

The for...in loop in JavaScript can also be used to iterate through the elements of an array. However, it is not recommended to do this as this is less efficient than using a for...of loop.

Syntax

The syntax of for...in loop in JavaScript is as follows −

for (variableName in object) {
   statement or block to execute
}

Parameters

• variableName − It is a property name (key) of the object.

• in − It is an 'in' operator in JavaScript.

• object − It is the object to traverse.

In each iteration, one property from object is assigned to variableName and this loop continues till all the properties of the object are exhausted.

Examples

Try the following examples to implement 'for-in' loop.

Example: Iterate through object properties

In the example below, the car object contains various properties. We used the forin loop to traverse through each key of the object.

In the output, we can see that it prints the key and its value. We use the '[]' (member of) operator to access the value of the key from the object.

<html>
<head>
   <title> JavaScript - for...in loop </title>
</head>
<body>
   <p id = "output"> </p>
   <script>
      let output = document.getElementById("output");
      let car = {
         brand: "OD",
         model: "Q7",
         color: "Black",
      }
      for (key in car) {
         output.innerHTML += key + " -> " + car[key] + "<br>";
      }
    </script>
</body>
</html>

Output

brand -> OD
model -> Q7
color -> Black

Example: Iterating over a string

In JavaScript, the string is an object. So, we can use the forin loop to traverse through each character of the string. The index of the character is the key, and the character is a value.

The code prints the index and character in the output.

<html>
<head>
   <title> JavaScript - for...in loop </title>
</head>
<body>
   <p id = "output"> </p>
   <script>
      let output = document.getElementById("output");
      let str = "Hello";
      for (key in str) {
         output.innerHTML += key + " -> " + str[key] + "<br>";
      }
   </script>
</body>
</html>

Output

0 -> H
1 -> e
2 -> l
3 -> l
4 -> o

Exampl: Iterating over an array

In JavaScript, the array is also an object. So, the forin loop can be used to traverse through array elements. Like a string, the index is a key, and the array element is a value for the key.

The below code prints the array index and its value in the output.

<html>
<head>
   <title> JavaScript - for...in loop </title>
</head>
<body>
   <p id = "output"> </p>
   <script>
      let output = document.getElementById("output");
      let array = ["Hi", "Hello", 900, 23, true, "JavaScript"];
      for (key in array) {
         output.innerHTML += key + " -> " + array[key] + "<br>";
      }
   </script>
</body>
</html>

Output

0 -> Hi
1 -> Hello
2 -> 900
3 -> 23
4 -> true
5 -> JavaScript

Example: Update value of each property of an object

In the example below, we traverse each key of the object and update its value to null. In the output, the code prints the object keys with null values.

So, the forin loop can also be used to update all or particular property values of the object.

<html>
<head>
    <title> JavaScript - for...in loop </title>
</head>
<body>
    <p id = "output"> </p>
    <script>
        let output = document.getElementById("output");
        let car = {
            brand: "OD",
            model: "Q7",
            color: "Black",
        }
        for (key in car) {
            car[key] = null;
        }
        output.innerHTML += "The updated object is - " + JSON.stringify(car);
    </script>
</body>
</html>

Output

The updated object is - {"brand":null,"model":null,"color":null}

Example: Iterating the Browser's Navigator Object

Try the following example to implement 'for-in' loop. It prints the web browsers Navigator object.

<html>
<body>
  <div id = "demo"> </div>
  <script>
    const output = document.getElementById("demo");
    var aProperty;
    output.innerHTML = "Navigator Object Properties<br> ";        
    for (aProperty in navigator) {
      output.innerHTML += aProperty;
      output.innerHTML += "<br>";
    }
    output.innerHTML += "Exiting from the loop!";
  </script>      
</body>
</html>

Output

Navigator Object Properties
vendorSub
productSub
vendor
maxTouchPoints
userActivation
doNotTrack
geolocation
connection
plugins
mimeTypes
pdfViewerEnabled
webkitTemporaryStorage
webkitPersistentStorage
hardwareConcurrency
cookieEnabled
appCodeName
appName
appVersion
platform
product
userAgent
language
languages
onLine
webdriver
getBattery
getGamepads
javaEnabled
sendBeacon
vibrate
scheduling
bluetooth
clipboard
credentials
keyboard
managed
mediaDevices
storage
serviceWorker
wakeLock
deviceMemory
ink
hid
locks
mediaCapabilities
mediaSession
permissions
presentation
serial
virtualKeyboard
usb
xr
userAgentData
canShare
share
clearAppBadge
setAppBadge
getInstalledRelatedApps
getUserMedia
requestMIDIAccess
requestMediaKeySystemAccess
webkitGetUserMedia
registerProtocolHandler
unregisterProtocolHandler
Exiting from the loop!