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JavaScript - Array Destructuring

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Array Destructuring

JavaScript Array destructuring allows us to unpack the values from array and assign them to the variables. After that, you can use the variables to access their value and use them in the code. We can perform the array structuring using the destructuring assignment. The destructuring assignment is a basically a JavaScript expression.

Syntax

The syntax of array destructuring assignment in JavaScript is as follows – 

const array = [10, 20];
const [var1, var2] = array;

In the above syntax, the "array" is the original array containing 10 and 20. When you unpack the array elements, var1 contains the 10, and var2 contains the 20.

Example: Basic Array Destructuring

In the example below, the arr array contains 3 numeric values. After that, we used the array destructuring to unpack array elements and store them in the num1, num2, and num3 variables.

<html>
<body>
   <div id = "output1"> </div>
   <div id = "output2"> </div>
   <div id = "output3"> </div>
   <script>
  
      const arr = [100, 500, 1000];
    
      const [num1, num2, num3] = arr;
    
      document.getElementById("output1").innerHTML = "num1: " + num1;
      document.getElementById("output2").innerHTML = "num2: " + num2;
      document.getElementById("output3").innerHTML = "num3: " + num3;
    
   </script>
</body>
</html>

Output

num1: 100
num2: 500
num3: 1000

Example: Unpacking with initial N elements of the array

While destructuring the array, if the left operand has fewer variables than array elements, first, N array elements get stored in the variables.

The array contains 6 elements in the below code, but 2 variables exist on the left side. So, the first 2 elements of the array will be stored in the variables, and others will be ignored.

<html>
<body>
   <div id = "output1"> </div>
   <div id = "output2"> </div>
   <script>
      const arr = [1, 2, 3, 4, 5, 6];
      const [num1, num2] = arr;
       
      document.getElementById("output1").innerHTML = "num1: " + num1;
      document.getElementById("output2").innerHTML = "num2: " + num2;
   </script>
</body>
</html>

Output

num1: 1
num2: 2

Skipping Array Elements While Destructuring an Array

While JavaScript array destructuring, you can skip a particular array element without assigning it to a variable.

Example

In the below code, the arr array contains 6 numbers. We have skipped the 2nd and 4th elements. In the output, num3 is 3, and num5 is 5, as the 2nd and 4th elements are skipped.

<html>
<body>
   <div id = "output"> </div>
   <script>
      const arr = [1, 2, 3, 4, 5, 6];
      const [num1, , num3, , num5] = arr;
      document.getElementById("output").innerHTML = 
		"num1: " + num1 + "<br>" +
      "num3: " + num3 + "<br>" +
      "num5: " + num5;
   </script>
</body>
</html>

Output

num1: 1
num3: 3
num5: 5

Array Destructuring and Rest Operator

If the left operand of the destructuring assignment operator contains fewer variables than the array elements, JavaScript ignores the last remaining array elements.

To solve the problem, you can use the rest operator. You can write the last variable with a rest operator in the left operand. So, the remaining array elements will be stored in the operand of the rest operator in the array format.

Example

In the code below, the array's first and second elements are stored in the num1 and num2 variables. We used the rest operator with the num3 variable. So, the remaining array elements will be stored in num3 in the array format.

<html>
<body>
   <div id = "demo"> </div>
   <script>
      const arr = [1, 2, 3, 4, 5, 6];
      const [num1, num2, ...nums] = arr;
      document.getElementById("demo").innerHTML = 
      "num1: " + num1 + "<br>" + 
      "num2: " + num2 + "<br>" +
      "nums: " + nums;
   </script>
</body>
</html>

Output

num1: 1
num2: 2
nums: 3,4,5,6
If you use the rest operator in the middle, all remaining array elements will be stored in the operand of the rest operator, and variables used after the rest operator will be ignored.

Array Destructuring and Default Values

Sometimes, an array can contain undefined values or fewer elements than the variables. In such cases, while destructuring JavaScript arrays, the variables can be initialized with the default values.

Example

In the below code, num1, num2, and num3 contain 10, 20, and 30 default values, respectively. The array contains only a single element.

So, when we unpack the array, the num1 variable will contain the array element, num2 and num3 will contain the default values.

<html>
<body>
   <div id = "demo"> </div>
   <script>
      const arr = [1];
      const [num1 = 10, num2 = 20, num3 = 30] = arr;
      document.getElementById("demo").innerHTML = 
      "num1: " + num1 + "<br>" +
      "num2:  " + num2 + "<br>" +
      "num3: " + num3;
   </script>
</body>
</html>

Output

num1: 1
num2: 20
num3: 30

Swapping Variables Using Array Destructuring

You can also swap two variables’ values using the JavaScript Array Destructuring.

Example

In the below code, variables a and b are initialized with 50 and 100, respectively. After that, we added variables a and b in a different order in the left and right operands. In the output, you can see the swapped values of the a and b.

<html>
<body>
   <div id = "output"> </div>
   <script>
      let a = 50, b = 100;
      document.getElementById("output").innerHTML = 
      "Before Swapping: a = " + a + ", b = " + b + "<br>";
      [b, a] = [a, b]
      document.getElementById("output").innerHTML += 
      "After Swapping: a = " + a + ", b = " + b;
   </script>
</body>
</html>

Output

Before Swapping: a = 50, b = 100
After Swapping: a = 100, b = 50

Destructuring the Returned Array from the Function

In real-time development, dynamic arrays are returned from the function. So, you can also destructure the returned array from the function.

Example

In the example below, getNums() function returns the array of numbers.

We execute the getNums() function and unpack array elements in the individual variables.

<html>
<body>
   <div id = "output"> </div>
   <script>
      function getNums() {
         return [99, 80, 70];
      }
      const [num1, num2, num3] = getNums();
      document.getElementById("output").innerHTML = 
      "num1: " + num1 + "<br>" +
      "num2: " + num2 + "<br>" +
      "num3: " + num3;
   </script>
</body>
</html>

Output

num1: 99
num2: 80
num3: 70
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