Arrays in JavaScript/TypeScript also have these kind of methods. You can again filter with you condition and then use reduce aggregation function to sum the items.

Copyconst sum = receiptItems.filter(item => item.tax === '25.00')
                        .reduce((sum, current) => sum + current.total, 0);

item.tax === '25.00' - this part you must adjust with your logic

I know that this question has an accepted answer but I thought I'd chip in with an alternative which uses array.reduce, seeing that summing an array is the canonical example for reduce:

Copy$scope.sum = function(items, prop){
    return items.reduce( function(a, b){
        return a + b[prop];
    }, 0);
};

$scope.travelerTotal = $scope.sum($scope.traveler, 'Amount');

Fiddle

You can do it with reduce:

this.sum = arrItem.reduce((acc, val) => {
  if (val.id !== 2) return acc + val.value;
  return acc;
}, 0);

To make sure that a number is interpreted as a number in JavaScript, use the parseFloat(object) method when printing the total and the Number(object) method when retrieving them into the method, like so;

getCompletedWork(){

    var total:number = 0
    this.state.items.forEach(item => {
        total += Number(item.hours) + (Number(item.mins)/60)
        console.log("total is : ", parseFloat(total))
    });

return total
}

This will explicitly ensure that the numbers are treated as numbers and not text strings.

Use Array.prototype.reduce(), the reduce() method applies a function against an accumulator and each value of the array (from left-to-right) to reduce it to a single value.

var array = [{
  "adults": 2,
  "children": 3
}, {
  "adults": 2,
  "children": 1
}];

var val = array.reduce(function(previousValue, currentValue) {
  return {
    adults: previousValue.adults + currentValue.adults,
    children: previousValue.children + currentValue.children
  }
});
console.log(val);