"head" returns undefined if source is empty or not array

src/head/head.js

@@ -12,6 +12,7 @@
  * head([1, 2, 3])
  * // => 1
  * head([])
- * // => null
+ * // => undefined
  */
-module.exports = source => (source.length === 0 ? null : source[0])
+module.exports = source =>
+  Array.isArray(source) && source.length !== 0 ? source[0] : undefined

src/head/head.test.js

@@ -15,12 +15,14 @@ const head = require("./head")
  * head([1, 2, 3])
  * // => 1
  * head([])
- * // => null
+ * // => undefined
  */
 test("array::head", t => {
-  t.equal(head([1, 2, 3]), 1, "Get first element of number array")
-
-  t.equal(head([]), null, "Get first element of empty array")
+  t.equal(head([1, 2, 3]), 1, "From number array should return first element")
+  t.equal(head([]), undefined, "From empty array should return undefined")
+  t.equal(head(2), undefined, "From number should return undefined")
+  t.equal(head({}), undefined, "From object should return undefined")
+  t.equal(head(() => {}), undefined, "From function should return undefined")
 
   t.end()
 })