Better way to sum a property value in an array

Updated Answer

Due to all the downsides of adding a function to the Array prototype, I am updating this answer to provide an alternative that keeps the syntax similar to the syntax originally requested in the question.

class TravellerCollection extends Array {
    sum(key) {
        return this.reduce((a, b) => a + (b[key] || 0), 0);
    }
}
const traveler = new TravellerCollection(...[
    {  description: 'Senior', Amount: 50},
    {  description: 'Senior', Amount: 50},
    {  description: 'Adult', Amount: 75},
    {  description: 'Child', Amount: 35},
    {  description: 'Infant', Amount: 25 },
]);

console.log(traveler.sum('Amount')); //~> 235

Original Answer

Since it is an array you could add a function to the Array prototype.

traveler = [
    {  description: 'Senior', Amount: 50},
    {  description: 'Senior', Amount: 50},
    {  description: 'Adult', Amount: 75},
    {  description: 'Child', Amount: 35},
    {  description: 'Infant', Amount: 25 },
];

Array.prototype.sum = function (prop) {
    var total = 0
    for ( var i = 0, _len = this.length; i < _len; i++ ) {
        total += this[i][prop]
    }
    return total
}

console.log(traveler.sum("Amount"))

The Fiddle: http://jsfiddle.net/9BAmj/

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:

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

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

Fiddle

Just another take, this is what native JavaScript functions Map and Reduce were built for (Map and Reduce are powerhouses in many languages).

var traveler = [{description: 'Senior', Amount: 50},
                {description: 'Senior', Amount: 50},
                {description: 'Adult', Amount: 75},
                {description: 'Child', Amount: 35},
                {description: 'Infant', Amount: 25}];

function amount(item){
  return item.Amount;
}

function sum(prev, next){
  return prev + next;
}

traveler.map(amount).reduce(sum);
// => 235;

// or use arrow functions
traveler.map(item => item.Amount).reduce((prev, next) => prev + next);

Note: by making separate smaller functions we get the ability to use them again.

// Example of reuse.
// Get only Amounts greater than 0;

// Also, while using Javascript, stick with camelCase.
// If you do decide to go against the standards, 
// then maintain your decision with all keys as in...

// { description: 'Senior', Amount: 50 }

// would be

// { Description: 'Senior', Amount: 50 };

var travelers = [{description: 'Senior', amount: 50},
                {description: 'Senior', amount: 50},
                {description: 'Adult', amount: 75},
                {description: 'Child', amount: 35},
                {description: 'Infant', amount: 0 }];

// Directly above Travelers array I changed "Amount" to "amount" to match standards.

function amount(item){
  return item.amount;
}

travelers.filter(amount);
// => [{description: 'Senior', amount: 50},
//     {description: 'Senior', amount: 50},
//     {description: 'Adult', amount: 75},
//     {description: 'Child', amount: 35}];
//     Does not include "Infant" as 0 is falsey.

I always avoid changing prototype method and adding library so this is my solution:

Using reduce Array prototype method is sufficient

// + operator for casting to Number
items.reduce((a, b) => +a + +b.price, 0);

I thought I'd drop my two cents on this: this is one of those operations that should always be purely functional, not relying on any external variables. A few already gave a good answer, using reduce is the way to go here.

Since most of us can already afford to use ES2015 syntax, here's my proposition:

const sumValues = (obj) => Object.keys(obj).reduce((acc, value) => acc + obj[value], 0);

We're making it an immutable function while we're at it. What reduce is doing here is simply this: Start with a value of 0 for the accumulator, and add the value of the current looped item to it.

Yay for functional programming and ES2015! :)

You can do the following:

$scope.traveler.map(o=>o.Amount).reduce((a,c)=>a+c);

Alternative for improved readability and using Map and Reduce:

const traveler = [
    {  description: 'Senior', amount: 50 },
    {  description: 'Senior', amount: 50 },
    {  description: 'Adult', amount: 75 },
    {  description: 'Child', amount: 35 },
    {  description: 'Infant', amount: 25 },
];

const sum = traveler
  .map(item => item.amount)
  .reduce((prev, curr) => prev + curr, 0);

Re-useable function:

const calculateSum = (obj, field) => obj
  .map(items => items.attributes[field])
  .reduce((prev, curr) => prev + curr, 0);

It's working for me in TypeScript and JavaScript:

let lst = [
     { description:'Senior', price: 10},
     { description:'Adult', price: 20},
     { description:'Child', price: 30}
];
let sum = lst.map(o => o.price).reduce((a, c) => { return a + c });
console.log(sum);

I hope is useful.

I'm not sure this has been mentioned yet. But there is a lodash function for that. Snippet below where value is your attribute to sum is 'value'.

_.sumBy(objects, 'value');
_.sumBy(objects, function(o) { return o.value; });

Both will work.

can also use Array.prototype.forEach()

let totalAmount = 0;
$scope.traveler.forEach( data => totalAmount = totalAmount + data.Amount);
return totalAmount;

Here is a one-liner using ES6 arrow functions.

const sumPropertyValue = (items, prop) => items.reduce((a, b) => a + b[prop], 0);

// usage:
const cart_items = [ {quantity: 3}, {quantity: 4}, {quantity: 2} ];
const cart_total = sumPropertyValue(cart_items, 'quantity');

How to sum array of object using Javascript

const traveler = [
  {  description: 'Senior', Amount: 50},
  {  description: 'Senior', Amount: 50},
  {  description: 'Adult', Amount: 75},
  {  description: 'Child', Amount: 35},
  {  description: 'Infant', Amount: 25 }
];

const traveler = [
    {  description: 'Senior', Amount: 50},
    {  description: 'Senior', Amount: 50},
    {  description: 'Adult', Amount: 75},
    {  description: 'Child', Amount: 35},
    {  description: 'Infant', Amount: 25 },
];
function sum(arrayData, key){
   return arrayData.reduce((a,b) => {
  return {Amount : a.Amount + b.Amount}
})
}
console.log(sum(traveler))

I was already using jquery. But I think its intuitive enough to just have:

var total_amount = 0; 
$.each(traveler, function( i, v ) { total_amount += v.Amount ; });

This is basically just a short-hand version of @akhouri's answer.

Here's a solution I find more flexible:

function sumOfArrayWithParameter (array, parameter) {
  let sum = null;
  if (array && array.length > 0 && typeof parameter === 'string') {
    sum = 0;
    for (let e of array) if (e && e.hasOwnProperty(parameter)) sum += e[parameter];
  }
  return sum;
}

To get the sum, simply use it like that:

let sum = sumOfArrayWithParameter(someArray, 'someProperty');

From array of objects

function getSum(array, column)
  let values = array.map((item) => parseInt(item[column]) || 0)
  return values.reduce((a, b) => a + b)
}

foo = [
  { a: 1, b: "" },
  { a: null, b: 2 },
  { a: 1, b: 2 },
  { a: 1, b: 2 },
]

getSum(foo, a) == 3
getSum(foo, b) == 6