Getting the Sum of a Particular Field for the Objects in an Array in JavaScript [duplicate]

Question

I know I can use reduce to sum the numbers of an array, like so:

let numArray = [1, 2, 3, 4].reduce((a, b) => a + b, 0);
console.log(numArray);

However, if I have an array of objects, of which one field's value is a number, how would I go about getting the sum of just that field for the objects in the array?

Say for instance for this kind of data structure:

[
  {
    name: "John Smith",
    balance: 42
  },
  {
    name: "Jane Doe",
    balance: 24
  }
]

How would I use reduce to get the sum of just the "balance" field for all the objects in the array?

use b.balance

ASDFGerte
– ASDFGerte

2018-06-29 14:58:44 +00:00
Commented Jun 29, 2018 at 14:58 — ASDFGerte
– ASDFGerte, Commented Jun 29, 2018 at 14:58

Ivan · Accepted Answer · 2018-06-29 15:30:10Z

6

Try the following:

var arr = [{name: "John Smith",balance: 42},{name: "Jane Doe",balance: 24}];

var result = arr.reduce((a,curr) => a + curr.balance, 0);

console.log(result);

You can even make a generic function for this like :

var arr = [{name: "John Smith",balance: 42, average : 20},{name: "Jane Doe",balance: 24,  average : 30}];

function getSumFromObjects(arr,prop) {
  return arr.reduce((a,curr) => a + curr[prop], 0);
}
  
//get sum of prop balance
console.log(getSumFromObjects(arr,"balance"));

//get sum of prop average
console.log(getSumFromObjects(arr,"average"));

edited Jun 29, 2018 at 15:30

Ivan

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answered Jun 29, 2018 at 14:59

amrender singh

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1 Comment

Matteo Gaggiano Over a year ago

if balance is undefined or not a number format the result will be 042undefined24

Matteo Gaggiano · Accepted Answer · 2018-06-29 15:06:32Z

0

Always sanitize the numeric value of an object or you can have some unexpected results:

var arr = [{
  name: "John Smith",
  balance: 42
}, {
  name: "Jane Doe",
  balance: 24
}, {
  name: "Jane Doe Str",
  balance: "24"
}, {
  name: "Jane Smith Str",
  balance: "42"
}];

var okResult = arr.reduce((a, curr) => a + (curr.balance ? parseInt(curr.balance) : 0), 0);

var wrongResult = arr.reduce((a, curr) => a + curr.balance, 0);


console.log("Ok: ", okResult);
console.log("Wrong: ", wrongResult);

answered Jun 29, 2018 at 15:06

Matteo Gaggiano

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1 Comment

ASDFGerte Over a year ago

If the data is assured to contain a number for the value of the property (which will almost always be the case, data formats are rarely defined by users), i don't see a reason to sanitize. Also, if it is necessary, your way is not proper at all - parseInt("32nd street") === 32 but if that string appears, an exception should definitely be thrown. In case it's something completely different, NaN will be useless aswell. Having NaN creep into later calculations will usually just create terrible bugs.

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