Suppose you have two arrays:

const first = ['one', 'two']
const second = ['three', 'four']

and you want to merge them into one single array

How can you do so?

The modern way is to use the destructuring operator, to create a brand new array:

const result = [...first, ...second]

This is what I recommend. Note that this operator was introduced in ES6, so older browsers (read: Internet Explorer) might not support it.

If you want a solution that works also with older browsers, you could use the concat() method which can be called on any array:

const result = first.concat(second)

Both methods will generate a new array, without modifying the existing ones.