Extra elements being added to an array

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I was attempting to complete this problem as a newbie, and I'm seeing extra elements within my returned array after my loop.

In DNA strings, symbols "A" and "T" are complements of each other, as "C" and "G". 
You have function with one side of the DNA, you need to get the other complementary side.

Here is my solution

let dnaComStrand = [];
const DNAStrand = function(dnaStrand) {
  for (let i = 0; i < dnaStrand.length; i++) {
    if (dnaStrand[i] === `A`) {
      dnaComStrand[i] = `T`;
    } else if (dnaStrand[i] === `T`) {
      dnaComStrand[i] = `A`;
    } else if (dnaStrand[i] === `C`) {
      dnaComStrand[i] = `G`;
    } else if (dnaStrand[i] === `G`) {
      dnaComStrand[i] = `C`;
    }
  }
  //   return dnaComStrand.join(``);
  return dnaComStrand;
};
console.log(DNAStrand(`GCATA`));

yet in the console I was seeing

Array(5) [ "C", "G", "T", "A", "T" ]

which became even more confusing if I tried to call the function again.

let dnaComStrand = [];
const DNAStrand = function(dnaStrand) {
  for (let i = 0; i < dnaStrand.length; i++) {
    if (dnaStrand[i] === `A`) {
      dnaComStrand[i] = `T`;
    } else if (dnaStrand[i] === `T`) {
      dnaComStrand[i] = `A`;
    } else if (dnaStrand[i] === `C`) {
      dnaComStrand[i] = `G`;
    } else if (dnaStrand[i] === `G`) {
      dnaComStrand[i] = `C`;
    }
  }
  //   return dnaComStrand.join(``);
  return dnaComStrand;
};
console.log(DNAStrand(`GCATA`));
console.log(DNAStrand([`T`, `C`, `G`]));

which resulted in

Array(5) [ "A", "G", "C", "A", "T" ]

Where are these extra elements coming from?

arrays javascript
2 Answers

You can do:

const hash = {
  A: 'T',
  T: 'A',
  C: 'G',
  G: 'C',
}

const DNAStrand = dnaStrand => dnaStrand.split('').map(c => hash[c])

// 1
const dnaComStrand1 = DNAStrand(`GCATA`)
console.log(dnaComStrand1)

// 2
const dnaComStrand2 = DNAStrand(dnaComStrand1.join(''))
console.log(dnaComStrand2)

user_19325520

Reinitialize the array with new set of elements inside the function and before the loop.

user_16323050
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