Fix requiredArgs error message

src/_lib/requiredArgs/index.js

@@ -1,9 +1,12 @@
 export default function requiredArgs(required, args) {
   if (args.length < required) {
     throw new TypeError(
-      required + ' argument' + required > 1
-        ? 's'
-        : '' + ' required, but only ' + args.length + ' present'
+      required +
+        ' argument' +
+        (required > 1 ? 's' : '') +
+        ' required, but only ' +
+        args.length +
+        ' present'
     )
   }
 }

src/_lib/requiredArgs/test.js

@@ -0,0 +1,37 @@
+// @flow
+/* eslint-env mocha */
+
+import assert from 'power-assert'
+import requiredArgs from '.'
+
+describe('requiredArgs', function() {
+  function wrapperFn(required) {
+    // $ExpectedMistake
+    return function() {
+      requiredArgs(required, arguments)
+    }
+  }
+  const twoArgsRequired = wrapperFn(2)
+
+  context('with correct number of passed arguments', function() {
+    it('does not throw an error', function() {
+      assert.doesNotThrow(() => twoArgsRequired(1, 2))
+    })
+  })
+
+  context('with wrong number of arguments', function() {
+    it('throws correct error message', function() {
+      assert.throws(
+        function() {
+          twoArgsRequired(1)
+        },
+        function(err) {
+          return (
+            err instanceof TypeError &&
+            err.message === '2 arguments required, but only 1 present'
+          )
+        }
+      )
+    })
+  })
+})