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JavaScript JavaScript Array Iteration Methods Combining Array Methods Nested Data

Anik Devaughn
Anik Devaughn
7,751 Points

Challenge Task : Using the reduce method, extract all the customer hobbies into their own array. Store the hobbies

Hi, The Challenge ask for reduce method, and I tried using both map and reduce methods. Can I get some HINT what and where I have made mistakes with this code, Thanks in Advance!!! My code: const customers = [ { name: "Tyrone", personal: { age: 33, hobbies: ["Bicycling", "Camping"] } }, { name: "Elizabeth", personal: { age: 25, hobbies: ["Guitar", "Reading", "Gardening"] } }, { name: "Penny", personal: { age: 36, hobbies: ["Comics", "Chess", "Legos"] } } ]; let hobbies;

// hobbies should be: ["Bicycling", "Camping", "Guitar", "Reading", "Gardening", "Comics", "Chess", "Legos"] // Write your code below hobbies = customers .map(customer => customer.hobbies.map(hobby => hobby.hobbies)) .reduce ((arr, hobbies) => [...arr, ...hobbies], []); //console.log(hobbies);

app.js
const customers = [
  {
    name: "Tyrone",
    personal: {
      age: 33,
      hobbies: ["Bicycling", "Camping"]
    }
  },
  {
    name: "Elizabeth",
    personal: {
      age: 25,
      hobbies: ["Guitar", "Reading", "Gardening"]
    }
  },
  {
    name: "Penny",
    personal: {
      age: 36,
      hobbies: ["Comics", "Chess", "Legos"]
    }
  }
];
let hobbies;

// hobbies should be: ["Bicycling", "Camping", "Guitar", "Reading", "Gardening", "Comics", "Chess", "Legos"]
// Write your code below
hobbies = customers
  .map(customer => customer.hobbies.map(hobby => hobby.hobbies))
  .reduce ((arr, hobbies) => [...arr, ...hobbies], []);
//console.log(hobbies);
Manish Giri
Manish Giri
16,266 Points

In my .reduce(), I tried it with .concat() instead of array destructuring, and it worked. Maybe try that.

7 Answers

Juliette Tworsey
seal-mask
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Juliette Tworsey
Front End Web Development Techdegree Graduate 32,425 Points

Hi Anik!

In case you haven't solved this yet:

You are really close. There was one thing that you missed was when you were attempting to iterate over to the embedded hobbies array: you first need to get to the personal key/property before you can traverse further down into the embedded hobbies property/array.

Your map method should look like this:

.map(customer => customer.personal.hobbies.map(hobby => hobby))

Than you can initiate the reduce method like so:

.reduce((arr, hobbies) => [ ...arr, ...hobbies], [] );

I hope that this helps. This one had me stumped for a bit too.

Cheers:-)

Thank you Juliette,

This was a tricky one for me as well, than after checking out the code I noticed that the results are in an array, inside an array, inside another array :O

Steven Parker
Steven Parker
231,269 Points

Actually the hobbies are in an array that is inside an object that is inside another array. :wink:

Thanks Juliette!

i didn't think to map before using the reduce() method.

Hi Juliette! I know this was answered a long time ago so not sure if you're still around but I was wondering! This was my initial code:

hobbies = customers
  .map(user => user.personal.map(hobby => hobby.hobbies))
  .reduce((arr, allHobbies) => [...arr, ...allHobbies], []);

I tried to follow what he had been doing in the previous video, which seemed closer to that (accessing person, then using map to access hobbies). Is it bc he was accessing an object in an array and I am accessing an array within an object? Thankful for your answer as I would have been stuck! But would appreciate some insight if you (or anyone!) has the time and energy to educate!

Thanks again!

I had to read the MDN docs on array.prototype.reduce() to figure out the answer for this problem. There is a relevant section labeled "Bonding arrays contained in an array of objects using the spread operator and initialValue". The documentation can be found here https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/Reduce?v=a

If reduce is written like this, it can't find "hobbies":

.reduce ((arr, hobbies) => [...arr, ...hobbies], []);

Instead, write the reduce method like this:

.reduce((prev, curr) => [...prev, ...curr.personal.hobbies], []);
Mike Hatch
Mike Hatch
14,940 Points

Thanks, Mary Hawley. I had the answer similar to your first option. It makes sense that the code in your second option is the solution. Otherwise, .hobbies can't be accessed. All of the Challenges in this course were quite brain-draining! Will take some time to really wrap my head around these array methods, especially with .reduce.

Noah Yasskin
Noah Yasskin
23,947 Points
hobbies = customers
    .map(customer => customer.personal.hobbies.map(hobby => hobby))
    .reduce((arr, hobbies) => [...arr, ...hobbies], []);
Steven Parker
Steven Parker
231,269 Points

The nested "map" functions aren't necessary (and causing syntax errors). Much like the previous challenge, the "reduce" by itself is enough to solve the challenge, but this time you need to extract the correct properties from each item.

I'll bet you can get it now without an explicit spoiler.

Sarah Krabacher
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Sarah Krabacher
Full Stack JavaScript Techdegree Graduate 16,097 Points

Where did folks learn about using the ellipsis prior to acc and curr? I attempted to write my solution as hobbies = customers.reduce((acc, curr) => [acc, curr.personal.hobbies], []); I don't remember seeing anything in the instructional videos that mentions ellipsis. I'm wondering if I missed something? If anyone has a good resource for explaining the use of ... in this context, please share! I did a little bit of googling but struggled to find a resource that helped me understand.

Steven Parker
Steven Parker
231,269 Points

Depending on where they are used, ellipses might be for Rest parameters or the Spread syntax. The colored words link to the associated MDN pages on each.

Also, the Treehouse course Introducing ES2015 includes a video lesson on Rest Parameters and Spread Operator.

Sarah Krabacher
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.a{fill-rule:evenodd;}techdegree seal-36
Sarah Krabacher
Full Stack JavaScript Techdegree Graduate 16,097 Points

Thank you for the Treehouse video link Steven Parker ! Much appreciated :) As I continued through the course, I ended up watching a video that came AFTER the challenge and used spread syntax: https://teamtreehouse.com/library/nested-data-and-additional-exploration

Juliette Tworsey
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Juliette Tworsey
Front End Web Development Techdegree Graduate 32,425 Points

Hi Chelsea!

When I run this code:

hobbies = customers.map(user => user.personal.map(hobby => hobby.hobbies))
         .reduce((arr, allHobbies) => [...arr, ...allHobbies], []);

...I get this error:

TypeError: user.personal.map is not a function

What this is trying to say is that the map function` will not map over an object (in this case, user.personal is an object) directly. The map method is meant to iterate over arrays, so in order to iterate over the hobbies field, try accessing the field with a more direct approach like so:

hobbies = customers.map(user => user.personal.hobbies.map(hobby => hobby))
            .reduce((arr, allHobbies) => [...arr, ...allHobbies], []);

Notice that I mapped over user.personal.hobbies (because hobbies is an array) & I also removed hobbies from map(hobby => hobby.hobbies)). The main thing to remember is that the map method is meant to work with arrays.

I hope that this helps and makes sense.

Cheers:-)