array.splice()

a.splice(start, deleteCount, [item1, item2, …])

Changes the contents of an array by removing existing elements and/or inserting new ones.

Name	Type	Required	Default	Description
start	number	yes	—	Index at which to start changing the array.
deleteCount	number	yes	—	How many elements to remove from start.
items…	any	optional	—	Elements to insert at start.

let a = [1,2,3,4,5];
a.splice(1, 2, 100, 101, 102);
// a === [1,100,101,102,4,5]