I am supposed to write a function that takes an array of numbers and returns a string. The function is supposed to use filter, sort AND reduce to return a string containing only odd numbers, separated by commas in ascending order.

I can't seem to get my .reduce method to actually return anything and my if conditions don't seem to work correctly... (for example, it adds a comma and space after the output of the last number like this "3, 17, ")

Here is the code I have made so far...

const numSelectString = (numArr) => {
    // use .filter to select only the odd numbers
  numArr.filter((num) => num % 2 !== 0)
  // use. sort to sort the numbers in ascending order
  .sort((a,b) => a-b)
  // use reduce to create the string and put the commas and spaces in
  .reduce((acc, next, index) => {
    
    if (index 1!==undefined) {
      return acc = acc   next   ', ';;
    } else {
      return acc   next;
    }
  },'')
}

const nums = [17, 34, 3, 12]
console.log(numSelectString(nums)) // should log "3, 17"

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Does anyone have time to help me sort out my reduce method? I tried to pass in the index to determine if reduce is iterating on the last element to not output the ", " after the last element... but haven't got it to work... Also, I still don't understand why I keep getting "undefined" as my output!

Thanks for any help!

CodePudding user response：

You could replace the check with just taking the addition and without a start value.

const
    numSelectString = numArr => numArr
        .filter((num) => num % 2 !== 0)
        .sort((a, b) => a - b)
        .reduce(
            (acc, next, index) => index
                ? acc   ', '   next
                : next,
            ''
        );

console.log(numSelectString([17, 34, 3, 12]));
console.log(numSelectString([2, 4, 6]));
console.log(numSelectString([1, 2]));

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CodePudding user response：

Instead of using index 1!==undefined, try this

const numSelectString = (numArr) => {

  const numArrLength = numArr.length;
  const arr = [...numArr];

  arr
    .filter((num) => num % 2 !== 0)
    .sort((a,b) => a - b)
    .reduce((acc, next, index) => {
       if (index < numArrLength - 1) return acc  = `${next}, `;
       else return acc   `${next}`;
    }, "")

return arr;
}

It is also considered a good practice to not mutate the parameters.

Another method

Using the .join() method makes it way easy.

const numSelectString = (numArr) => {

const arr = [...numArr]
  arr
    .filter((num) => num % 2 !== 0)
    .sort((a,b) => a-b)
    .join(", ");

return arr;
}