Versions of collectSymbols

collectSymbols.js

const input = ['AED_AFN', 'ALL_AMD', 'AMD_AFN', 'ALL_GBP'];                                                                                                                                                
console.log('input:', JSON.stringify(input));                                                                                                                                                              
                                                                                                                                                                                                           
// somewhat functional, somewhat imperative                                                                                                                                                                
const collectSymbols0 = pairs => {                                                                                                                                                                         
  const collectedSymbols = {};                                                                                                                                                                             
                                                                                                                                                                                                           
  pairs                                                                                                                                                                                                    
    .map(pair => pair.split('_'))                                                                                                                                                                          
    .forEach(([left, right]) => {                                                                                                                                                                          
      if (!collectedSymbols[left]) {                                                                                                                                                                       
        collectedSymbols[left] = [];                                                                                                                                                                       
      }                                                                                                                                                                                                    
      collectedSymbols[left].push(right);                                                                                                                                                                  
    });                                                                                                                                                                                                    
                                                                                                                                                                                                           
  return collectedSymbols;                                                                                                                                                                                 
};                                                                                                                                                                                                         
console.log('collectSymbols0(input):', JSON.stringify(collectSymbols0(input)));                                                                                                                           
                                                                                                                                                                                                           
                                                                                                                                                                                                           
// more functional, less imperative                                                                                                                                                                        
const collectSymbols1 = pairs => {                                                                                                                                                                         
  const collectedSymbols = {};                                                                                                                                                                             
                                                                                                                                                                                                           
  pairs                                                                                                                                                                                                    
    .map(pair => pair.split('_'))                                                                                                                                                                          
    .forEach(([left, right]) => (collectedSymbols[left] = collectedSymbols[left] || []).push(right));                                                                                                      
                                                                                                                                                                                                           
  return collectedSymbols;                                                                                                                                                                                 
};                                                                                                                                                                                                         
                                                                                                                                                                                                           
console.log('collectSymbols1(input):', JSON.stringify(collectSymbols1(input)));                                                                                                                           
                                                                                                                                                                                                           
                                                                                                                                                                                                           
// functional, imperative                                                                                                                                                                                  
const collectSymbols2 = pairs => pairs                                                                                                                                                                     
      .map(pair => pair.split('_'))                                                                                                                                                                        
      .reduce((collectedSymbols, [left, right]) => {                                                                                                                                                       
        collectedSymbols[left] = collectedSymbols[left] || [];                                                                                                                                             
        collectedSymbols[left].push(right);                                                                                                                                                                
        return collectedSymbols;                                                                                                                                                                           
      }, {});                                                                                                                                                                                              
console.log('collectSymbols2(input):', JSON.stringify(collectSymbols2(input)));                                                                                                                           
                                                                                                                                                                                                           
// fairly functional, slightly imperative                                                                                                                                                                
const collectSymbols3 = pairs => pairs                                                                                                                                                                     
      .map(pair => pair.split('_'))                                                                                                                                                                        
      .reduce((collectedSymbols, [left, right]) => ((collectedSymbols[left] = collectedSymbols[left] || []).push(right), collectedSymbols), {});                                                           
console.log('collectSymbols3(input):', JSON.stringify(collectSymbols3(input)));     

// separate imperative from functional                                                                                                                                                                     
const pushy = (collectedSymbols, [left, right]) => ((collectedSymbols[left] = collectedSymbols[left] || []).push(right), collectedSymbols);
const collectSymbols4 = pairs => pairs.map(pair => pair.split('_')).reduce(pushy, {});
console.log('collectSymbols4(input):', JSON.stringify(collectSymbols4(input)));

/* $ node collectSymbols.js
input: ["AED_AFN","ALL_AMD","AMD_AFN","ALL_GBP"]
collectSymbols0(input): {"AED":["AFN"],"ALL":["AMD","GBP"],"AMD":["AFN"]}
collectSymbols1(input): {"AED":["AFN"],"ALL":["AMD","GBP"],"AMD":["AFN"]}
collectSymbols2(input): {"AED":["AFN"],"ALL":["AMD","GBP"],"AMD":["AFN"]}
collectSymbols3(input): {"AED":["AFN"],"ALL":["AMD","GBP"],"AMD":["AFN"]}
collectSymbols4(input): {"AED":["AFN"],"ALL":["AMD","GBP"],"AMD":["AFN"]}
*/