ECMAScript 6 sets: union, intersection, difference
Check out my book (free online): “Exploring ES6”. Updated version of this blog post: section “Union, intersection, difference”.
A recent question by Dmitry Moskowski reminded me: ECMAScript 6 sets have no methods for computing the union (e.g. addAll), intersection (e.g. retainAll) or difference (e.g. removeAll). This blog post explains how to work around that limitation.
Union
Union (a ∪ b): create a set that contains the elements of both set a and set b.
let a = new Set([1,2,3]);
let b = new Set([4,3,2]);
let union = new Set([...a, ...b]);
// {1,2,3,4}
The pattern is always the same:
- Convert one or both sets to arrays.
- Perform the operation.
- Convert the result back to a set.
As explained in [1], the spread operator (...) inserts the elements of something iterable (like a set) into an array. Therefore, [...a, ...b] means that a and b are converted to arrays and concatenated. It is equivalent to [...a].concat([...b]).
Intersection
Intersection (a ∩ b): create a set that contains those elements of set a that are also in set b.
let a = new Set([1,2,3]);
let b = new Set([4,3,2]);
let intersection = new Set(
[...a].filter(x => b.has(x)));
// {2,3}
Steps: Convert a to an array, filter the elements, convert the result to a set.
Difference
Difference (a \ b): create a set that contains those elements of set a that are not in set b. This operation is also sometimes called minus (-).
let a = new Set([1,2,3]);
let b = new Set([4,3,2]);
let difference = new Set(
[...a].filter(x => !b.has(x)));
// {1}
Conclusion
This blog post showed how you can implement union, intersection and different for sets. Long-term, I expect JavaScript to have built-in functionality for this, e.g. via functions that operate on iterables (similar to Python’s itertools).
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