JavaScript Symbol isConcatSpreadable Property

JavaScript Symbol.isConcatSpreadable is a well-known symbol used to configure if a given object should be flattened to its array elements while using the Array.prototype.concat() method.
Syntax:

Array[Symbol.isConcatSpreadable]

Here Array is the array object which are to be flattened to its array elements.
Parameters: This symbol does not accept any parameter.
Return value: This symbol does not return any value.
JavaScript code to show the working of this function. 
Example-1:

javascript

<script>
   // Creating some arrays
   const Array1 = [1, 2, 3];
   const Array2 = [4, 5, 6];

   // Calling concat() function
   let Array3 = Array1.concat(Array2);

   // Printing the concatenated array
   console.log(Array3);

   // Calling Symbol.isConcatSpreadable symbol
   Array2[Symbol.isConcatSpreadable] = false;
   Array3 = Array1.concat(Array2);

   // Printing the concatenated array
   // after calling of Symbol.isConcatSpreadable symbol
   console.log(Array3);
</script>

Output:

> Array [1, 2, 3, 4, 5, 6]
> Array [1, 2, 3, Array [4, 5, 6]]

Example-2:

javascript

<script>
   // Creating some arrays
   const Array1 = [1, 2, 3];
   const Array2 = [4, 5, 6];

   // Calling concat() function
   let Array3 = Array1.concat(Array2);

   // Printing the concatenated array
   console.log(Array3);

   // Calling Symbol.isConcatSpreadable symbol
   Array2[Symbol.isConcatSpreadable] = true;
   Array3 = Array1.concat(Array2);

   // Printing the concatenated array
   // after calling of Symbol.isConcatSpreadable symbol
   console.log(Array3);
</script>

Output:

> Array [1, 2, 3, 4, 5, 6]
> Array [1, 2, 3, 4, 5, 6]

Supported Browsers:

• Google Chrome 48 and above
• Firefox 48 and above
• Edge 15 and above
• Opera 35 and above
• Apple Safari 10 and above