Update snippets head and last to type check

Update `head` and `last` to check on empty, undefined, null arrays.

snippets/head.md

@@ -5,12 +5,16 @@ tags: array,beginner
 
 Returns the head of a list.
 
-Use `arr[0]` to return the first element of the passed array.
+Check if `arr` is truthy and has a `length` property, use `arr[0]` if possible
+to return the first element, otherwise return `undefined`
 
 ```js
-const head = arr => arr[0];
+const head = arr => (arr && arr.length ? arr[0] : undefined);
 ```
 
 ```js
 head([1, 2, 3]); // 1
-```
+head([]); // undefined
+head(null); // undefined
+head(undefined); // undefined
+```

snippets/last.md

@@ -5,12 +5,17 @@ tags: array,beginner
 
 Returns the last element in an array.
 
-Use `arr.length - 1` to compute the index of the last element of the given array and returning it.
+Check if `arr` is truthy and has a `length` property, use `arr.length - 1` to
+compute the index of the last element of the given array and returning it,
+otherwise return `undefined`
 
 ```js
-const last = arr => arr[arr.length - 1];
+const last = arr => (arr && arr.length ? arr[arr.length - 1] : undefined);
 ```
 
 ```js
 last([1, 2, 3]); // 3
-```
+last([]); // undefined
+last(null); // undefined
+last(undefined); // undefined
+```

test/head.test.js

@@ -15,20 +15,14 @@ test('head({ 0: false}) returns false', () => {
 test('head(String) returns S', () => {
   expect(head('String')).toBe('S');
 });
-test('head(null) throws an Error', () => {
-  expect(() => {
-    head(null);
-  }).toThrow();
+test('head(null) returns undefined', () => {
+  expect(head(null)).toBe(undefined);
 });
-test('head(undefined) throws an Error', () => {
-  expect(() => {
-    head(undefined);
-  }).toThrow();
+test('head(undefined) returns undefined', () => {
+  expect(head(undefined)).toBe(undefined);
 });
-test('head() throws an Error', () => {
-  expect(() => {
-    head();
-  }).toThrow();
+test('head() returns undefined', () => {
+  expect(head()).toBe(undefined);
 });
 let start = new Date().getTime();
 head([1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 1122, 32124, 23232]);

test/last.test.js

@@ -15,20 +15,14 @@ test('last({ 0: false}) returns undefined', () => {
 test('last(String) returns g', () => {
   expect(last('String')).toBe('g');
 });
-test('last(null) throws an Error', () => {
-  expect(() => {
-    last(null);
-  }).toThrow();
+test('last(null) returns undefined', () => {
+  expect(last(null)).toBe(undefined);
 });
-test('last(undefined) throws an Error', () => {
-  expect(() => {
-    last(undefined);
-  }).toThrow();
+test('last(undefined) returns undefined', () => {
+  expect(last(undefined)).toBe(undefined);
 });
-test('last() throws an Error', () => {
-  expect(() => {
-    last();
-  }).toThrow();
+test('last() returns undefined', () => {
+  expect(last()).toBe(undefined);
 });
 let start = new Date().getTime();
 last([1, 2, 3, 4, 5, 6, 7, 8, 9, 11, 1122, 32124, 23232]);