How to append or concat strings in Javascript?

Question

So I\'m trying to add to a string and it shows up as empty.

var DNA = \"TAG\";
var mRNA = \"\";

m_RNA()

function check(a, b, string) {
  if (string = a) {
             


        

Answer 1

mRNA.concat(b); doesn't mutate the string, it only computes the value. You need tomRNA = mRNA.concat(b) (or mRNA = mRNA + b) to change the value of mRNA.

Answer 2

Rather than trying to mutate variables on the upper scope, you could consider to wrap your task into a function...

let's refactor it making sure m_RNA returns the correct mapping:

var DNA = "TAG";

// create a map so every char maps to something.
const map = {
  T: 'A',
  A: 'U',
  C: 'G',
  G: 'C',
};

// check only needs to pick from map, or return an empty string.
function check(fragment) {
  return map[fragment] || '';
}


function m_RNA(dna) {
  // reduces all the chars to the new sequence
  return Array.from(dna).reduce(
    function (result, fragment) {
      return result.concat(check(fragment))
    },
    "",
  );
}

var mRNA = m_RNA(DNA);
console.log('mRNA', mRNA);