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.

const xx = [1];
console.log(JSON.stringify(xx.concat(2)) === "[1,2]");
// true

// original unchanged
console.log(JSON.stringify(xx) === "[1]");
// true

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