# 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:

## 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.

Could you clarify what SCT stands for??

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

//versus

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?

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;`