parse-request-body.js

// input
// [
//   'pref-001|choice-001',
//   'pref-001|choice-002',
//   'pref-002|choice-001',
// ]


// output
// [
//   { reference: 'pref-001', choices: ['choice-001', 'choice-002'] },
//   { reference: 'pref-002', choices: ['choice-001'] },
// ]


// solution: 3 stages
//
// 1. reduce [] into {}
// [                                                 {
//   'pref-001|choice-001',                      =>    'pref-001': ['choice-001', 'choice-002'],
//   'pref-001|choice-002',                            'pref-002': ['choice-001'],
//   'pref-002|choice-001',                          }
// ]
//
// 2. Object.entries() to turn {} => [] so I can map
// {                                                 [
//   'pref-001': ['choice-001', 'choice-002'],   =>    ['pref-001', ['choice-001', 'choice-002']]
//   'pref-002': ['choice-001'],                       ['pref-002', ['choice-001']]
// }                                                 ]
//
// 3. map [[]] => [{}]
// [                                                 [
//   ['pref-001', ['choice-001', 'choice-002']]  =>    { reference: 'pref-001', choices: ['choice-001', 'choice-002'] },
//   ['pref-002', ['choice-001']]                      { reference: 'pref-002', choices: ['choice-001'] },
// ]                                                 ]

const output = 
    Object.entries(
        input.reduce((output, preference) => {
            const [reference, choice] = preference.split('|');

            return {
                ...output,
                [reference]: [...output[reference] || [], choice],
            };
        }, {})
    )
    .map(([reference, choices]) => ({ reference, choices }));