mentoring.md

Mentoring

This exercise focuses on

• String iteration (String#replace, String#split)
• Object/Map

Reasonable solutions

const TRANSCRIPTION = {
  C: 'G',
  G: 'C',
  A: 'U',
  T: 'A',
};

export function toRna(sequence) {
  return sequence
    .split('')
    .map(nucleotide => TRANSCRIPTION[nucleotide])
    .join('')
}

Variations include Set or Map with #get, which are valid!

String destructuring can also be used:

return [...sequence].map(/**/).join('')

String#replace can also be used:

return sequence.replace(/./g, (nucleotide) => /* */)

Variations include replacing only the "known" nucleotides:

sequence.replace(/[CGAT]/g, (nucleotide) => /* */)

Note

There are versions that check for valid Input DNA (which has since been removed), meaning they include the following or variations:

function invalidInput() {
  throw new Error('Invalid input DNA.')
}

/*...*/
  .map(nucleotide => TRANSCRIPTION[nucleotide] || invalidInput())
/*...*/

Since the error handling is now unnecessary, it will be considered defensive programming. The student does not need to validate the input, as the tests no longer ask for it.

Common suggestions

• Use an Object to keep track of the mapping instead of conditionals.
• Use iteration via String#split or String#replace instead of using for/forEach with Array#push
• Discourage Array#reduce for this particular solution, because it creates a lot of intermediary strings (more than the split approach), except if the rest of the solution is correct (then you can mention it but approve). Using reduce requires more interpretation by the reader to follow, change and maintain.
• Discourage String#substring with foreach iteration, because character iteration via split('') is more idiomatic and maintainable than substring with 1. Using split('') requires less interpretation by the reader to follow, change and maintain.

Talking points

• Encourage streams because it's harder to use intermediary variables (which can lead to bugs) and forces "one-by-one" approach. It is easier to explain the Object solution over multiple conditionals when it's a stream.
• If a student uses an Object, point them to falsy values.