functional programming, pure functions

Author: CoreyScript

Assets/PureFunctions.png

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+# Functional Programming
+
+Functional programming (often abbreviated FP) is the process of building software by composing pure functions, avoiding shared state, mutable data, and side-effects. Functional programming is declarative rather than imperative, and application state flows through pure functions. Contrast with object oriented programming, where application state is usually shared with methods in objects.
+
+<i>[Reference Article](https://medium.com/javascript-scene/master-the-javascript-interview-what-is-functional-programming-7f218c68b3a0)</i>
+
+## Pure Functions
+
+![Pure Functions](./Assets/PureFunctions.png)
+
+If you want to break things down in functional programming it all comes down to this concept of pure functions. The idea is that there's a separation between data over a program and the behavior of a program. All objects created in functional programming are immutable, so after it has been created it cannot be changed.
+
+Rules:
+
+- The function has to always return the same output, given the same input
+- The function cannot modify anything outside of itself
+- No side effects
+
+<i>Example:</i>
+
+```javascript
+const array = [1,2,3] // in the outside world (don't modify)
+
+function removeLastItem(arr) {
+    const newArray = [].concat(arr); // make a copy of the input instead of directly modifying it
+    newArray.pop()
+    return newArray
+}
+
+function multiplyByTwo(arr) {
+    return arr.map(item => item * 2) // using .map() will return a new array
+}
+
+const array2 = removeLastItem(array);
+const array3 = multiplyByTwo(array);
+
+console.log(array, array2, array3) // ->  [1, 2, 3] , [1, 2] , [2, 4, 6]
+```
+
+#### <i>Example:</i> Amazon shopping feature
+
+```javascript
+
+```