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JavaScript JavaScript Array Iteration Methods Array Manipulation Return a Single Value from an Array with reduce()

Posted by Daniel Cranney
Daniel Cranney
11,608 Points

I got the right answer for numberOf503 challenge, but it feels like a cheat. How else could I do it?

So here's my solution for the numberOf503 challenge. It gives the right result (3), but it only really checks for a 5 at index 1, and a 3 at index 3. If the number at index 2 was different, it would still get added to my reduce result.

How else could I have done this, in a more reliable way?

const phoneNumbers = ["(503) 123-4567", "(646) 123-4567", "(503) 987-6543", "(503) 234-5678", "(212) 123-4567", "(416) 123-4567"];
let numberOf503;

// numberOf503 should be: 3
// Write your code below

numberOf503 = phoneNumbers.reduce( (count, number) => {
  if(number[1] === '5' && number[3] === '3') {
   return count + 1 
  }
  return count;
 }, 0);

console.log(numberOf503);

1 Answer

Cody Hansen
Cody Hansen
5,517 Points

There are a few ways to answer this! If you're just looking for numbers that start with 503, you could use substrings or regular expressions!

With substrings, instead of looking for "5" at index 1 and "3" at index 3, you would just be looking for "(503)" at the start of the string. Like this!:

let numberOf503;

// numberOf503 should be: 3
// Write your code below

numberOf503 = phoneNumbers.reduce( (count, number) => {
  if(number.substring(0,5) === "(503)") {
   return count + 1; 
  }
  return count;
 }, 0);

This webpage goes over the method very thoroughly if you would like some extra reading: https://www.w3schools.com/jsref/jsref_substring.asp

With regular expressions, you're also looking for "503" at the beginning of the string by using a list of rules to follow. Like this!:

let numberOf503;

// numberOf503 should be: 3
// Write your code below
const regex = /\(?503.*/m;

numberOf503 = phoneNumbers.reduce((count, phone) => {
  if(regex.test(phone)) {
    return count += 1;
  }
  else {
    return count;
  }
}, 0);

I would go more into regular expressions, but they are a different beast of rules all on their own. There are courses here on Treehouse about them. I highly recommend them! They are very useful.

I hope this helps!

Daniel Cranney
Daniel Cranney
11,608 Points

Thanks Cody Hansen, I knew the solution would likely be a simple one but I didn't think to use substring in that way for some reason. Great solution, really helpful and well explained too!