const arr1 = [

  {"id": "5c5030b9a1ccb11fe8c321f4", "quantity": 1},

  {"id": "344430b94t4t34rwefewfdff", "quantity": 5},

  {"id": "342343343t4t34rwefewfd53", "quantity": 3}

];

const arr2 = [

  {"id": "5c5030b9a1ccb11fe8c321f4", "quantity": 2},

  {"id": "344430b94t4t34rwefewfdff", "quantity": 1}

];

​

const result = Object.values([...arr1, ...arr2].reduce((acc, { id, quantity }) => {

  acc[id] = { id, quantity: (acc[id] ? acc[id].quantity : 0) + quantity  };

  return acc;

}, {}));

​

console.log(result);

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