add closure exercise

Author: emkis

exercises/closure/README.md

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+# Closure
+
+This is an exercise to practice closure.
+
+## Instructions
+
+1. Modify `strBuilder(..)` so that it can take a string and return back a function.
+
+	**Note:** For purposes of this exercise, assume that `strBuilder(..)` itself is always called with a string initially.
+
+2. For each call to a function here, if a string is passed, a function should be returned.
+
+3. If a non-string is passed (such as passing no argument), a string value should be returned, which is the concatenation of all the passed in strings.
+
+4. Hints:
+	- You can use `typeof foo == "string"` to test if `foo` is a string.
+
+	- Look at the test cases at the bottom of the exercise file to clarify any questions about expected behavior.
+
+	- Ensure your function(s) are pure. Avoid mutating a closed over variable, which would be a side-effect.

exercises/closure/initial-exercise.js

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+'use strict'
+
+function strBuilder (str) {
+  return strBuilder
+}
+
+var hello = strBuilder('Hello, ')
+var kyle = hello('Kyle')
+var susan = hello('Susan')
+var question = kyle('?')()
+var greeting = susan('!')()
+
+console.log(strBuilder('Hello, ')('')('Kyle')('.')('')() === 'Hello, Kyle.')
+console.log(hello() === 'Hello, ')
+console.log(kyle() === 'Hello, Kyle')
+console.log(susan() === 'Hello, Susan')
+console.log(question === 'Hello, Kyle?')
+console.log(greeting === 'Hello, Susan!')