freeCodeCamp/curriculum/challenges/english/02-javascript-algorithms-and-data-structures/functional-programming/avoid-mutations-and-side-effects-using-functional-programming.english.md

id, title, challengeType, forumTopicId

id	title	challengeType	forumTopicId
587d7b8e367417b2b2512b5e	Avoid Mutations and Side Effects Using Functional Programming	1	301228

Description

If you haven't already figured it out, the issue in the previous challenge was with the splice call in the tabClose() function. Unfortunately, splice changes the original array it is called on, so the second call to it used a modified array, and gave unexpected results. This is a small example of a much larger pattern - you call a function on a variable, array, or an object, and the function changes the variable or something in the object. One of the core principles of functional programming is to not change things. Changes lead to bugs. It's easier to prevent bugs knowing that your functions don't change anything, including the function arguments or any global variable. The previous example didn't have any complicated operations but the splice method changed the original array, and resulted in a bug. Recall that in functional programming, changing or altering things is called mutation, and the outcome is called a side effect. A function, ideally, should be a pure function, meaning that it does not cause any side effects. Let's try to master this discipline and not alter any variable or object in our code.

Instructions

Fill in the code for the function incrementer so it returns the value of the global variable fixedValue increased by one.

Tests

tests:
  - text: Your function <code>incrementer</code> should not change the value of <code>fixedValue</code> (which is <code>4</code>).
    testString: incrementer(); assert(fixedValue === 4);
  - text: Your <code>incrementer</code> function should return a value that is one larger than the <code>fixedValue</code> value.
    testString: const __newValue = incrementer(); assert(__newValue === 5);
  - text: Your <code>incrementer</code> function should return a value based on the global `fixedValue` variable value.
    testString: |
      (function() {
       fixedValue = 10;
       const newValue = incrementer();
       assert(fixedValue === 10 && newValue === 11);
       fixedValue = 4;
      })();

Challenge Seed

// The global variable
var fixedValue = 4;

function incrementer () {
  // Only change code below this line


  // Only change code above this line
}

Solution

var fixedValue = 4

function incrementer() {
  return fixedValue + 1
}