How do I calculate the difference between two lists in Javascript?

Consider these two lists:

let oldItems = [   1, 2, 3, 4   ];
let newItems = [0, 1, 2,       5];

Moving from oldItems to newItems, you see that:

• items 3 and 4 are removed
• items 0 and 5 are added

Here is the code that uses filter and some to calculate the inserts and deletes in Javascript:

let inserts = newItems.filter(x => !oldItems.some(y => x === y));
let deletes = oldItems.filter(x => !newItems.some(y => x === y));

console.log(inserts);
console.log(deletes);

result:

[ 0, 5 ]
[ 3, 4 ]

Object equality

If you want to create inserts and deletes for lists that contain more complex objects like class instances, you might want to consider replacing === with an equals function