To extend an existing array in JavaScript, use the Array.concat() method.
Example
You can try to run the following code to extend an existing JavaScript array with another array:
<html>
<head>
<title>JavaScript Array push Method</title>
</head>
<body>
<script>
var numbers = new Array(1, 4, 9);
var numbers2 = new Array(2, 5);
numbers = numbers.concat(numbers2);
document.write("new numbers is : " + numbers );
</script>
</body>
</html>Output
new numbers is : 1,4,9,2,5