11，arr.splice

var fruits = ["Banana", "Orange", "Apple", "Mango"];
fruits.splice(2,1,"Lemon","Kiwi");

// Banana,Orange,Lemon,Kiwi,Mango

index从“Banana”的左边0开始
1，表示要切除一个元素
然后在该空档处添加2个元素