bankFinder.js

const eligibleInstitutions = [
  {
    institution_name: 'BMO',
    institution_number: '001',
  },
  {
    institution_name: 'Simplii',
    institution_number: '010',
    transit_numbers: ['30800']
  },
  {
    institution_name: 'CIBC',
    institution_number: '010'
  },
  {
    institution_name: 'TD',
    institution_number: '004'
  }
]

const bankAccounts = [
  {
    institution_number: '001',
    transit_number: '12345',
    account_number: '1342237'
  },
  {
    institution_number: '004',
    transit_number: '34533',
    account_number: '1212347'
  },
  {
    institution_number: '004',
    transit_number: '35673',
    account_number: '7433453'
  },
  {
    institution_number: '010',
    transit_number: '30800',
    account_number: '9054343'
  }
]

// Takes an array of bank accounts, and returns an array of insitution names that match the array of bank accounts
function find(accounts, institutions) {
  let bankNames = new Set();

  accounts.forEach(account => {
    let result;

    // First, do a search for transit and institution.
    result = institutions.find(institution => {
      if (
        'transit_numbers' in institution && 
        institution.transit_numbers[0] === account.transit_number &&
        institution.institution_number === account.institution_number
      ) {
        return true;
      }
    });

    // If nothing found, do a search for institution only.
    if (result === undefined || result.length === 0) {
      result = institutions.find(institution => {
        if (institution.institution_number === account.institution_number) {
          return true;
        }
      })
    }

    if (result?.institution_name) {
      bankNames.add(result.institution_name);
    }

  });
  return Array.from(bankNames);
}

const result = find(bankAccounts, eligibleInstitutions);
const expectedAnswer = ['BMO', 'TD', 'Simplii'];

// We can't directly compare arrays in JS, so using JSON.stringify here.
console.log(`Is it working?: ${JSON.stringify(result) === JSON.stringify(expectedAnswer)}`);