JS: Array.prototype.concat

myArray.concat(new1, new2 …)

Return a new array.

myArray is not changed.

The new array will have arguments added to the end of the array myArray.

Each argument can be any type. If it is a array, it will be flattened 1 level (That is, remove the outmost brackets).

// example of using array.concat()

const mya = [1];

console.log ( mya.concat( 2, [3], 4, {k9:9}) );
// [ 1, 2, 3, 4, { k9: 9 } ]

console.log ( mya );
// [1]

To add without flatten, use push. See: JS: Array.prototype.push.

For how to flatten nested array, see: JS: Flatten Nested Array.

Reference

ECMAScript® 2016 Language Specification#sec-array.prototype.concat