Merge pull request #1050 from Bjvanminnen/compose

composition can be seeded with multiple arguments

src/utils/compose.js

@@ -6,5 +6,15 @@
  * left. For example, compose(f, g, h) is identical to arg => f(g(h(arg))).
  */
 export default function compose(...funcs) {
-  return arg => funcs.reduceRight((composed, f) => f(composed), arg)
+  return (...args) => {
+    if (funcs.length === 0) {
+      // We weren't given any functions, just return the first passed in arg.
+      return args[0]
+    }
+
+    const last = funcs[funcs.length - 1]
+    const rest = funcs.slice(0, -1)
+
+    return rest.reduceRight((composed, f) => f(composed), last(...args))
+  }
 }

test/utils/compose.spec.js

@@ -21,5 +21,17 @@ describe('Utils', () => {
       expect(compose(b, c, a)(final)('')).toBe('bca')
       expect(compose(c, a, b)(final)('')).toBe('cab')
     })
+
+    it('can be seeded with multiple arguments', () => {
+      const square = x => x * x
+      const add = (x, y) => x + y
+      expect(compose(square, add)(1, 2)).toBe(9)
+    })
+
+    it('returns the first given argument if given no functions', () => {
+      expect(compose()(1, 2)).toBe(1)
+      expect(compose()(3)).toBe(3)
+      expect(compose()()).toBe(undefined)
+    })
   })
 })