intersection (#5)

* intersection
* intersectionBy

src/index.ts

@@ -33,3 +33,4 @@ export * from './compose'
 export * from './difference'
 export * from './async'
 export * from './nth'
+export * from './intersection'

src/intersection.ts

@@ -0,0 +1,14 @@
+type by<T> = (item: T) => number | string
+
+export function intersection<T>(...args: Array<T[] | by<T>>): T[] {
+  if (!args || !args.length) return []
+  if (args.slice(0, args.length - 1).find(item => typeof item === 'function')) return []
+  const list: T[][] = args.filter(Array.isArray)
+  const by: by<T> = typeof args[args.length - 1] === 'function' ? args[args.length - 1] as by<T> : (x => x as number);
+  return list?.reduce((a, b) => {
+    const setB = new Set(b.map(by));
+    return a.filter(c => setB.has(by(c)));
+  });
+}
+
+export { intersection as intersectionBy }

test/intersection.spec.ts

@@ -0,0 +1,21 @@
+import { intersection } from '../src'
+import { intersectionBy } from '../src'
+
+describe('should work', function () {
+  let list = [['a', 'b', 'c'], ['a', 'c'], ['a'], ['a', 'd']]
+  it('should work', function () {
+    expect(intersection()).toEqual([])
+    expect(intersection([])).toEqual([])
+    expect(intersection([2, 1], [4, 2], [1, 2])).toEqual([2])
+    expect(intersection([2, 1])).toEqual([2, 1])
+    expect(intersection(...list)).toEqual(['a'])
+  })
+
+  it('should work with by', function () {
+    expect(intersectionBy()).toEqual([])
+    expect(intersectionBy([])).toEqual([])
+    expect(intersectionBy([{ 'x': 1 }])).toEqual([{ 'x': 1 }])
+    expect(intersectionBy([2, 1], [4, 2], [1, 2], Math.floor)).toEqual([2])
+    expect(intersectionBy([{ 'x': 1 }], [{ 'x': 2 }, { 'x': 1 }], x => x.x)).toEqual([{ 'x': 1 }])
+  })
+})