JS How to use reduce on array containing multiple numerical values

Question

I have an array like this.

[{
    PropertyOne : 1,
    PropertyTwo : 5
},
{
    PropertyOne : 3,
    PropertyTwo : 5
},...]

And I want to end up with an array like this which aggregates all the columns of this array to end up like this.

[{
    PropertyOne : 4,
    PropertyTwo : 10
}}

If it was a single column I know I could use .reduce but can't see how I could do with multiple columns ?

You have access to the full object in your reduce callback. Accumulate against each property and return the full object. — ste2425
– ste2425, Commented Oct 24, 2016 at 18:20

Dan O · Accepted Answer · 2016-10-24 18:20:16Z

16

var array = [{
  PropertyOne : 1,
  PropertyTwo : 5
},
{
  PropertyOne : 2,
  PropertyTwo : 5
}];
var reducedArray = array.reduce(function(accumulator, item) {
  // loop over each item in the array
  Object.keys(item).forEach(function(key) {
    // loop over each key in the array item, and add its value to the accumulator.  don't forget to initialize the accumulator field if it's not
    accumulator[key] = (accumulator[key] || 0) + item[key];
  });

  return accumulator;
}, {});

answered Oct 24, 2016 at 18:20

Dan O

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2 Comments

Explosion Pills Over a year ago

You can just wrap this in an array to get what you ask for specifically in your question (array of one object)

Dan O Over a year ago

@ExplosionPills , good point! I missed that. var reducedArray = [array.reduce( /*... */ )];

Joel H · Accepted Answer · 2021-06-24 18:27:47Z

7

The same (as other answers) using ES6 arrow functions:

    var reducedArray = array.reduce((accumulator, item) => {
      Object.keys(item).forEach(key => {
        accumulator[key] = (accumulator[key] || 0) + item[key];
      });
      return accumulator;
    }, {});

edited Jun 24, 2021 at 18:27

answered Jun 10, 2019 at 14:04

Joel H

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1 Comment

Jim Over a year ago

You could also wrap the accumulator with . if (typeof item[key] === 'number') { .... accumulatory[key].... }

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