How to merge two arrays and sum values of duplicate objects using lodash

Question

There are two arrays:

[
  {\"id\": \"5c5030b9a1ccb11fe8c321f4\", \"quantity\": 1},
  {\"id\": \"344430b94t4t34rwefewfdff\", \"quantity\": 5},
  {\"id\": \"34         


        

Answer 1

Version with additional object keys. The body of the function does not interfere with what object has properties. So sum by "qty" and check by "prop"

var first = [ 
  {quantity:100, id:1, variantId: 1}, 
  {quantity:300, id:2, variantId: 2, propA: 'aaa'},

];

var second = [
  {quantity:100, id:1, variantId: 1},
  {quantity:200, id:2, variantId: 2, propB: true}, 
  {quantity:300, id:3, variantId: 3, propC: 'ccc'}, 
]

function mergeArrays(arrayOfArrays, propToCheck, propToSum) {
    let sum = [];
    [].concat(...arrayOfArrays).map(function(o) {
        
        let existing = sum.filter(function(i) { return i[propToCheck] === o[propToCheck] })[0];
    
        if (!existing) {

            sum.push(o);
        } else {

            existing[propToSum] += o[propToSum];
            
            let copyProps = Object.keys(o).filter(obj => {
                  return existing[obj] !== o[obj]
            }).map(val => (val !== propToSum) ? existing[val] = o[val] : null)
        }
             
    });

    return sum;
}

console.log(mergeArrays([first, second], 'variantId', 'quantity'))