FAQ: Code Challenges: Intermediate JavaScript - sortYears()

This community-built FAQ covers the “sortYears()” exercise from the lesson “Code Challenges: Intermediate JavaScript”.

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FAQs on the exercise sortYears()

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const sortYears = arr => arr.sort((x, y) => y - x)

I don’t understand what " (x, y) => y - x " is doing, why would it return a year and not the value of y minus x?


I want to know that too, I don’t understand that… Can some one explain how does it work?

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because of how sort works, here is useful documentation:

we pass our own function to the sort method to determine sorting order. The result of y - x is then used to determine were the element should be sorted. for example, the first two elements in our array are:

1970, 1999

so if we then fill in our function which determine sorting order:

(1970, 1999) => 1999 - 1970

which gives a a value higher then 0, which means y comes before x.

in short, the .sort() method will call your function to determine sorting order.


Something to consider would be:

const sortYears = arrYears => {
  return arrYears.sort().reverse();

const years = [1970, 1999, 1951, 1982, 1963, 2011, 2018, 1922]

I suggest you first
console.log(years.sort()) ( inside the function it’s console.log(arrYears.sort()) )
you’ll notice the numbers in the array are descending from left to right.
then chain the reverse() method to log out the numbers ascending from left to right (in the correct order).

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true, but sorting them in the correct order in the first place is very likely more efficient. Programming is always a tricky balance (performance vs readability vs maintainability)

How does y-x work to define the sort order?

You have (1970, 1999) => 1999 - 1970, but how are the rest of the years impacted?

Mainly, I’m confused about the output that checkYears produces, for sort() to use. Does checkYears go one by one through all the entire array? Like 1999 - 1970, 1951 - 1999, 1982 - 1951, 1963 - 1982, etc.

From the solution:

// As a function declaration AND using a named function:
function sortYears(arr) {
const checkYears = (year1, year2) => year2 - year1
return arr.sort(checkYears)

the .sort() method implement logic under the hood, as explained here:


Yeah so I’ve literally solved this and am still getting marked wrong despite the fact that we aren’t told HOW this needs to be solved beyond using .sort(). Here’s my code:

The solution didn’t make sense to me either and even though I’ll read up on it, I’m frustrated that I’m getting marked wrong for how I went about getting my answer.

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It is showing wrong because of the instruction of the intermediate JavaScript Practice: "You’ll have to pass in your own function in .sort() method to get the functionality you want. "

That feeling :see_no_evil: when u spend one hour to try to figure out the response and suddenly u find out that u need to complete it in 2 lines of code.

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I referenced a different article regarding the .sort descending issue, after understanding what I needed to do, my output was this:

// Write your code here:

//callback function to sort array output in descending order
function sortDescending(a, b) {
  return a > b ? -1 : b > a ? 1 : 0;

// function to return array items
const sortYears = arr => {
  return years.sort(sortDescending); // uses .sort method and calls sortDescending to arrange elements in descending order

// Feel free to uncomment the below code to test your function:

const years = [1970, 1999, 1951, 1982, 1963, 2011, 2018, 1922]

// Should print [ 2018, 2011, 1999, 1982, 1970, 1963, 1951, 1922 ]

According to my console the output is correct, but the lesson isnt marked off as checked. Any ideas why?

the error is a good hint:

Does your function call sort() on the array that was passed into it?

where it means the sortYears function. If you added another array:

const years = [1970, 1999, 1951, 1982, 1963, 2011, 2018, 1922]
const temp = [1988, 1977, 1999, 2111, 2222, 1966]


i do not get the desired output for my temp array.

ahh I see, dammit same mistake I made a few lessons back.


return years.sort(sortDescending);


return arr.sort(sortDescending);

all is well. Thanks!

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God this exercise is killing me… youtube videos here i come

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With all the trouble I’ve been having with the intermediate java scrip challenges I deiced to start simple and straight forward using what I understand, well…thought I understood, and then go back and be more efficient .

const years = [1970, 1999, 1951, 1982, 1963, 2011, 2018, 1922]

function sortYears(arr){
let newArray = ;
newArray = arr.sort();

for (let i=0; i<newArray.length; i++){
let newArray2 = ;
newArray2 = newArray.pop();
return newArray2


the output is 2018. Why is the for loop iterating only once?

you nested return in your loop. Return does what it says, it returns/hands back data to the caller, signaling that the function is done executing

My solution:
const sortYears = array => array.sort().reverse()

How do we memorize or remember all these built-in methods such as .map or .sort and what they do?

the methods you will use regular you will remember (muscle memory), for the rest you can use documentation :slight_smile: