JavaScript Practice Lesson 10 of 21

Array set operations

Back in Phase 4 you used .filter() to narrow an array down by a condition. The condition can just as easily be "is this missing from another array?" - a.filter((item) => !b.includes(item)) keeps only the items of a that b doesn't have, which is exactly what a set difference is.

For duplicates, JavaScript's built-in Set does the work for you: a Set can only ever hold one copy of each value, and it remembers the order things were added in. Spread an array into a Set and back into an array - [...new Set(arr)] - and you've deduplicated it in one line, first occurrence kept, order preserved.

Your task: write diff(a, b), returning the items in array a that are NOT in array b, and dedupe(arr), returning arr with duplicates removed, keeping the order items first appeared in.

You'll practice:

Filtering one array against the contents of another
Deduplicating an array with a Set

// Write diff(a, b): items in array a that are NOT in array b.
function diff(a, b) {

}

// Write dedupe(arr): arr with duplicates removed, keeping first-occurrence order.
function dedupe(arr) {

}