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// http://stackoverflow.com/questions/39828497/sum-of-string-values-on-array-object/

console.clear();


let array = [
  {quantity: 1, amount: "24.99"},
  {quantity: 5, amount: "4.99"}
]

function sumProperty(arr, type) {
  return arr.reduce((total, obj) => {
    if (typeof obj[type] === 'string') {
      return total + Number(obj[type]);
    }
    return total + obj[type];
  }, 0);
}

let totalAmount = ( sumProperty(array, 'amount') ).toFixed(2); 
console.log(  totalAmount  ); // 29.98

let totalQuantity = sumProperty(array, 'quantity'); 
console.log(  totalQuantity  ); // 6