Add forEach vs. map section to JS questions (yangshun#5)

* Add forEach vs. map section to JS questions

* Update README.md

README.md

@@ -765,9 +765,44 @@ A closure is the combination of a function and the lexical environment within wh
 * https://developer.mozilla.org/en-US/docs/Web/JavaScript/Closures
 * https://medium.com/javascript-scene/master-the-javascript-interview-what-is-a-closure-b2f0d2152b36
 
-### Can you describe the main difference between a `forEach` loop and a `.map()` loop and why you would pick one versus the other?
+### Can you describe the main difference between a `.forEach` loop and a `.map()` loop and why you would pick one versus the other?
 
-TODO
+To understand the differences between the two, let's look at what each function does.
+
+**`forEach`**
+
+- Iterates through the elements in an array.
+- Executes a callback for each element.
+- Does not return a value.
+
+```js
+const a = [1, 2, 3];
+const doubled = a.forEach((num, index) => {
+  // Do something with num and/or index.
+});
+
+// doubled = undefined
+```
+
+**`map`**
+
+- Iterates through the elements in an array.
+- "Maps" each element to a new element by calling the function on each element, creating a new array as a result.
+
+```js
+const a = [1, 2, 3];
+const doubled = a.map(num => {
+  return num * 2;
+});
+
+// doubled = [2, 4, 6]
+```
+
+The main difference between `.forEach` and `.map()` is that `.map()` returns a new array. If you need the result, but do not wish to mutate the original array, `.map()` is the clear choice. If you simply need to iterate over an array, `forEach` is a fine choice.
+
+###### References
+
+* https://codeburst.io/javascript-map-vs-foreach-f38111822c0f
 
 ### What's a typical use case for anonymous functions?