.concat() takes an existing array, adds a value to it and returns a new array it takes a copy of the original array, adds to it any parameters passed and returns a new array. .concat() is the ...

既存の配列を変更せず、新しい配列を返します。 https://developer.mozilla.org/ja/docs/Web/JavaScript/Reference/Global_Objects/Array/concat push ...

The concat() method of {{jsxref("Array")}} instances is used to merge two or more arrays. This method does not change the existing arrays, but instead returns a new array.