javascript sum array of objects reduce

How to call reduce on an array of objects to sum their properties?

stackoverflow.com › questions › 5732043 › how-to-call-reduce-on-an-array-of-objects-to-sum-their-properties

A cleaner way to accomplish this is by providing an initial value as the second argument to reduce:

Copyvar arr = [{x:1}, {x:2}, {x:4}];
var result = arr.reduce(function (acc, obj) { return acc + obj.x; }, 0);
console.log(result);  // 7

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The first time the anonymous function is called, it gets called with (0, {x: 1}) and returns 0 + 1 = 1. The next time, it gets called with (1, {x: 2}) and returns 1 + 2 = 3. It's then called with (3, {x: 4}), finally returning 7.

This also handles the case where the array is empty, returning 0.

Answer from Casey Chu on Stack Overflow

Stack Overflow

stackoverflow.com › questions › 5732043 › how-to-call-reduce-on-an-array-of-objects-to-sum-their-properties

javascript - How to call reduce on an array of objects to sum their properties? - Stack Overflow

Top answer

1 of 16

496

A cleaner way to accomplish this is by providing an initial value as the second argument to reduce:

Copyvar arr = [{x:1}, {x:2}, {x:4}];
var result = arr.reduce(function (acc, obj) { return acc + obj.x; }, 0);
console.log(result);  // 7

Run code snippetEdit code snippet Hide Results Copy to answer Expand

This also handles the case where the array is empty, returning 0.

2 of 16

414

After the first iteration your're returning a number and then trying to get property x of it to add to the next object which is undefined and maths involving undefined results in NaN.

try returning an object contain an x property with the sum of the x properties of the parameters:

Copyvar arr = [{x:1},{x:2},{x:4}];

arr.reduce(function (a, b) {
  return {x: a.x + b.x}; // returns object with property x
})

// ES6
arr.reduce((a, b) => ({x: a.x + b.x}));

// -> {x: 7}

Explanation added from comments:

The return value of each iteration of [].reduce used as the a variable in the next iteration.

Iteration 1: a = {x:1}, b = {x:2}, {x: 3} assigned to a in Iteration 2

Iteration 2: a = {x:3}, b = {x:4}.

The problem with your example is that you're returning a number literal.

Copyfunction (a, b) {
  return a.x + b.x; // returns number literal
}

Iteration 1: a = {x:1}, b = {x:2}, // returns 3 as a in next iteration

Iteration 2: a = 3, b = {x:2} returns NaN

A number literal 3 does not (typically) have a property called x so it's undefined and undefined + b.x returns NaN and NaN + <anything> is always NaN

Clarification: I prefer my method over the other top answer in this thread as I disagree with the idea that passing an optional parameter to reduce with a magic number to get out a number primitive is cleaner. It may result in fewer lines written but imo it is less readable.

GitHub

gist.github.com › benwells › 0111163b3cccfad0804d994c70de7aa1

Using Array.reduce to sum a property in an array of objects · GitHub

var items = [ { name: 'Item 1', ... 0 } ]; var msgTotal = items.reduce(function(prev, cur) { return prev + (cur.amount - (cur.amount * cur.discount) ); }, 0); document.write('Total of Discounted Items:', msgTotal);...

Discussions

How to reduce and sum a nested object array?

// reduce product to sum via matching keys const productReducer = (sumObj, product) => { const current = sumObj[product.productName] || 0; return { ...sumObj, [product.productName]: current + product.total } } // reduce product, return [key, product] const hourMapper = ([key, hours]) => { return [key, hours.reduce(productReducer, {})]; } // restore [key, val] array to object const flattenKeyVal = (obj, keyVal) => { return { ...obj, [keyVal[0]]: keyVal[1] } } // obj -> [key,product] -> [key, reducedProduct] -> reducedObj const dayMapper = day => { return Object.entries(day).map(hourMapper).reduce(flattenKeyVal); } // put it all together for the days const mapDays = days => days.map(dayMapper); I believe this should work! Fun little challenge. Let me know if you have any questions. More on reddit.com

r/learnjavascript

May 3, 2021

javascript - reduce & sum array of objects entries - Stack Overflow

But I cannot find a way to also sum the Budget figures together and add it to the same entry. I suspect I need to perform a separate reduce function on a duplicate array and push the result into the res array using the forEach loop with Year as the key. Can anyone see a way of doing this in the same reduce function? ... When initializing a Year object ... More on stackoverflow.com

stackoverflow.com

javascript - Using reduce to calculate the sum of an array of objects - Stack Overflow

I am trying to compute the sum of the weight of an array of object with children, however I think I am doing it the wrong way. I am trying to present a list of parcels using a searchable dropdown p... More on stackoverflow.com

stackoverflow.com

June 13, 2019

[deleted by user]

Honestly, just use reduce if you need to go from a collection of stufff to a single value. If you are using it for other things, you are probably abusing it More on reddit.com

r/reactjs

160

November 2, 2022

Videos

youtube.com

How to Find Sum of An Array of Numbers in Javascript - YouTube

August 22, 2021