Я нашел следующий код, и я не знаю, в чем разница между A и B:
var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.splice(2,0,["Lemon", "Kiwi"]);
В
fruits.splice(...[2,0].concat(["Lemon", "Kiwi"]));
var fruits = ["Banana", "Orange", "Apple", "Mango"];
var A = fruits.splice(2, 0, ["Lemon", "Kiwi"]);
var B = fruits.splice(...[2, 0].concat(["Lemon", "Kiwi"]));
console.log(A)
console.log(B)