FAQ: Learn TDD With Mocha - Red To Green I

This community-built FAQ covers the “Red To Green I” exercise from the lesson “Learn TDD With Mocha”.

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This exercise can be found in the following Codecademy content:

Web Development

Learn JavaScript Unit Testing

FAQs on the exercise Red To Green I

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Why must we declare const Calculate = {}; as a constant in this example?


Must you declare it as a constant? It’s common practice to do so, but as far as JavaScript is concerned, there’s no requirement to use const. I would, and always do use const unless there’s a reason not to. Did you try something else, and receive an error?

1 Like

Yep, used let and had to change it to const to move on

That’s the SCT for the exercise, but there’s nothing syntacticly wrong with using let.

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Well, when one doesn’t know what one is doing, those types of issues take a hit on ones confidence. :sweat_smile:

Could you clarify what SCT stands for??

Thanks in advance

Submission Correctness Test. The writer of the exercise wrote the SCT to only accept const it would appear.

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As I said previously, it is a good idea to use const whenever possible. It keeps us from accidentally changing things we don’t want to change. Consider the following:

function blah(a, b) {
  return a + b;
console.log(blah(7,9)); // 16
// many lines of code
blah = 'eleventeen' // accidentally overwrote my function
console.log(blah) // eleventeen
//many more lines of code
// try to use the function later
console.log(blah(7, 9)); // TypeError: blah is not a function


const blahblah = (a, b) => a + b
console.log(blahblah(7, 9)); // 16

blahblah = 'eleventeen' // TypeError: Assignment to constant variable.

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Here is my index.js

// index.js
const Calculate = {
  sum(x) {
    const reducer = (previousValue, currentValue) => previousValue + currentValue;
    return x.reduce(reducer)
module.exports = Calculate;

This is index_test.js

const assert = require('assert');
const Calculate =  require('../index.js')

describe('Calculate', () => {
  describe('.sum',() => {
    it('returns the sum of of a list\'s items', () => {
      assert.equal(Calculate.sum([1,2,3]), 6);

The TDD is green (passing), but after pressing the ‘Check Work’ button, it still does not allow to pass.
It errors with

Did you program the .sum() method to return 6?

Can some one help please?

1 Like

For anyone in the future that runs across this, on this step they want you to return 6 no matter what (currently it is Step 4).

You will define the actual code to return the sum based on the numbers passed to the method a few steps later.

1 Like

Is there any particular reason why Phrase is capitalized? I am on the Fullstack Engineer track and, as far as I remember, we’re only supposed to capitalize classes or constructor functions. But Phrase is neither… Can someone please correct me if I am wrong?

Hi everyone, quick question here… Why do I need to pass a parameter in sum() ? (Here, in the exercise, I got an error because I didn’t pass a parameter.

However, after passing a (x) parameter, my test succeeded (passing). Here’s my test code.

const Calculate = { sum(cal) { return 6; } }; module.exports = Calculate;