JS: Array.prototype.concat

xArray.concat(new1, new2, etc)

• add items to the end of array.
• Return a new array.
• Each argument can be any type.
• If an argument is a array, it is flattened 1 level.

console.log([1].concat(2));
// [ 1, 2 ]

// if arg is an array, it is flattened 1 level
console.log([1].concat([2, [3]]));
// [ 1, 2, [ 3 ] ]