Adding values from objects in an array

This should do what you want:

var array = [
 { id: "cards", amount: 5 }, 
 { id: "shirts", amount: 3 },
 { id: "cards", amount: 2 }, 
 { id: "shirts", amount: 3 }
];
var result = array.reduce(function(entities, item) {
 entities[item.id] = (entities[item.id] || 0) + item.amount;
 return entities;
}, {})
console.log(result);

You would loop your array, check the id property for your target object, then enumerate and outer scope variable with the value stored in the amount property.

var totalShirts = 0;
var totalCards = 0;
for(var i = 0, len = array.length; i < len; i++){
 var entry = array[i];
 if(entry.id === "cards"){
 totalCards += entry.amount;
 }
 else if(entry.id === "shirts"){
 totalShirts += entry.amount;
 }
}
console.log("Total Cards: " + totalCards);
console.log("Total Shirts: " + totalShirts);

Here is an example that gets the total of each item

var array = [{id:"cards", amount: 5}, {id:"shirts", amount: 3}, {id:"cards", amount: 2}, {id:"shirts", amount: 3}];
var result = array.reduce(function(accumulator, current) {
 if (!(current.id in accumulator)) {
 accumulator[current.id] = current.amount;
 } else {
 accumulator[current.id] += current.amount;
 }
 
 return accumulator;
}, {});
console.log(result);

A simple forEach will do the trick:

var counts = {}
array.forEach(v => {
 counts[v.id] = (counts[v.id] || 0) + v.amount
})
console.log(counts)

will print:

{
 cards: 7
 shirts: 6
}

Here is a O(n) time solution.

var totals = new Object();
for(var i = 0;i < array.length;i ++) {
 var id = array[i].id;
 var amount = array[i].amount;
 if(totals[id] == undefined) {
 totals[id] = amount; 
 } else {
 totals[id] += amount;
 }
}
console.log(totals);

You can use for..of loop

var array = [{
 id: "cards",
 amount: 5
}, {
 id: "shirts",
 amount: 3
}, {
 id: "cards",
 amount: 2
}, {
 id: "shirts",
 amount: 3
}]
let res = {};
for (let {id,amount} of array) {
 if (!res.hasOwnProperty(id)) res[id] = 0;
 res[id] += amount;
}
console.log(res);

Use a for-loop to do this:

var totalCards = 0; 
var totalShirt = 0;
for (var i = 0; i < arr.length; i++) {
 if (arr[i].id === "cards") {
 totalCards += arr[i].amount;
 } else {
 totalShirt += arr[i].amount;
 }
}

Do the magic in for loop. This example should be general enough:

var array = [ {id:"cards", amount: 5}, {id:"shirts", amount: 3}, {id:"cards", amount: 2}, {id:"shirts", amount: 3} ];
var output = [];
 
for(var i of array) {
 if(!output[i.id]) {
 output[i.id] = 0;
 }
 output[i.id] += i.amount;
}
 
console.log(output);

You can use Array.forEach() to iterate over each element of the array. The total object is an associative array where the index is the id field of array element objects.

var array = [{ id: "cards", amount: 5 },
 { id: "shirts", amount: 3 },
 { id: "cards", amount: 2},
 { id: "shirts", amount: 3 }];
var total = {};
array.forEach(function (el) {
 if (total[el.id]) {
 total[el.id] += el.amount
 } else {
 total[el.id] = el.amount
 }
});
console.log(JSON.stringify(total));

You could use Array#reduce and sum the amount.

var array = [{ id: "cards", amount: 5 }, { id: "shirts", amount: 3 }, { id: "cards", amount: 2 }, { id: "shirts", amount: 3 }],
 result = array.reduce(function (r, a) {
 r[a.id] = (r[a.id] || 0) + a.amount;
 return r;
 }, {});
 
console.log(result);

You can use this code

if (!Object.keys) {
 Object.keys = function (obj) {
 var keys = [],
 k;
 for (k in obj) {
 if (Object.prototype.hasOwnProperty.call(obj, k)) {
 keys.push(k);
 }
 }
 return keys;
 };
}

then you can do this in older browsers as well:

var len = Object.keys(obj).length;

• javascript
• arrays
• javascript-objects