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How to truncate an array in JavaScript?

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Aug 30, 2023 pm 12:41 PM

如何在 JavaScript 中截断数组？

This tutorial will show you how to truncate an array in JavaScript. Truncating an array means removing some elements from the end and assigning the new values of the truncated array back to the actual array.

Sometimes we need to truncate the array. Suppose in a case, there is an array in which there are sorted (descending) elements, now we just need the first k sorted elements in the array and remove the else value. So we don't need to create additional array and modify it and assign it to the actual array.

There are many ways to truncate an array, let’s look at all of them.

grammar

The following is the syntax for truncating an array:

arr.splice(index, howmany); // the splice() method.
arr.length=idx; // array length property.

Herearr is the original array. idx is the index/position where we want to truncate the array. Index is the position of the element to be deleted, and number is the number of elements to be deleted from the array.

algorithm

STEP 1 - Create an array arr and add elements to it. Use the innerHTML attribute to display arr.
STEP 2 - Apply the splice() method to the array arr, passing index and howmany as arguments. Alternatively, we can apply arr.length=idx.
Step 3 - Display the truncated array using the innerHTML attribute.

Example 1

Use Array splice()method

In the following example, we use the array splice() method to truncate the array. We define an array with seven elements. We truncate elements outside position 3.

<html>
<head>
   <title>Example- Truncate an array in JavaScript</title>
</head>
<body>
   <h3 id="Truncate-an-Array-using-splice-Method">Truncate an Array using splice() Method-</h3>
   <p id="before">Array before truncate:<br></p>
   <p id="after">Array after truncate:<br></p>
   <script>
      let arr = ["kalyan", "ashis", "anand", "sonu", "tirupati", "nagar", "arjun"];
      document.getElementById('before').innerHTML+=arr
      arr.splice(4, arr.length);
      document.getElementById('after').innerHTML+=arr
   </script>
</body>
</html>

Example 2

Using the array length attribute

In this example, we truncate the array using the array length property. We just assign the required length to the array and its size will change according to the allocated size.

<html>
<head>
   <title>Example- Truncate an array in JavaScript</title>
</head>
<body>
   <h3 id="Truncate-an-Array-using-Array-length-property">Truncate an Array using Array length property-</h3>
   <p id="before">Array before truncate:<br></p>
   <p id="after">Array after truancate:<br></p>
   <script>
      let arr = ["kalyan", "ashis", "anand", "sonu", "tirupati", "nagar", "arjun"];
      document.getElementById('before').innerHTML+=JSON.stringify(arr)
      arr.length=4
      document.getElementById('after').innerHTML+=JSON.stringify(arr)
   </script>
</body>
</html>

Here we use arr.length=4 to change the array to size 4. It gets the first 4 elements and after that all elements are removed from the array.

It has a disadvantage, we can only get elements from the starting index 0 to a given size, unlike splicing, we can choose to define the starting index and the ending index.

Now you may have a question in mind. What if the size we allocate is larger than the array length? Then we get an undefined array element after the array size.

Example 3

In the following example, we define an array of length 7 and then allocate a size larger than the array length.

<html>
<body>
   <p id="result1"><p>
   <p id="result2"><p>
   <script>
      let arr = ["kalyan", "ashis", "anand", "sonu", "tirupati", "nagar", "arjun"];
      arr.length=15
      document.getElementById('result1').innerHTML=JSON.stringify(arr);
      document.getElementById('result2').innerHTML=arr[13];
   </script>
</body>
</html>

Here we change the array size from 7 to 15 without adding elements to the array. Note that elements from index 7 to 14 are empty.

Example 4

Use Array slice()method

In the following example, we use the array slice() method to truncate the array. array The slice() method does not change the original array, it only returns the elements According to the given argument. So we need to store the returned element in a variable.

We define an array containing seven elements. We truncate elements outside position 3.

<html>
<head>
   <title>Example- Truncate an array in JavaScript</title>
</head>
<body>
   <h3 id="Truncate-an-Array-using-slice-Method">Truncate an Array using slice() Method</h3>
   <p id="before">Array before truncate:<br></p>
   <p id="after">Array after truncate:<br></p>
   <script>
      let arr = ["kalyan", "ashis", "anand", "sonu", "tirupati", "nagar", "arjun"];
      document.getElementById('before').innerHTML+=arr
      arr = arr.slice(0, 4);
      document.getElementById('after').innerHTML+=arr
   </script>
</body>
</html>

Here arr = arr.slice(0, 4); 0 is the starting index of the array, 4 is the ending index, until we want to splice the array, all elements before the given starting index and after the ending index are avoided and we then assign the updated array to the actual array.

If arr = arr.slice(2, 5); will be there then output will be
Anand, sonu, tirupati

In this tutorial, we learned how to truncate an array in JavaScript using the array splice() and slice() methods and the length property.

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