Sum all data in array of objects into new array of objects

Question

I have an array of objects that looks like this:

var data = [{costOfAirtickets: 2500, costOfHotel: 1200},{costOfAirtickets: 1500, costOfHotel: 1000}]
         


        

Answer 1

Another solution would be to use Map (not Array.prototype.map) as it has several notable differences compared to objects:

var data = [{
  costOfAirtickets: 2500,
  costOfHotel: 1200
}, {
  costOfAirtickets: 1500,
  costOfHotel: 1000
}]

let sums = data.reduce((collection,rcd)=>{
  Object.entries(rcd).forEach(([key,value])=>{
      let sum = collection.get(key) || 0
      collection.set(key, sum + +value)
  })
  return collection
}, new Map())

console.log(...sums.entries())

Explanation

Outer loop

The above first iterates over your data array using the reduce method. Each object within that I'll be referring to as a record -- distinguished in the code via the variable, rcd.

Each iteration of reduce returns a value which is passed as the first argument to the next iteration of the loop. In this case, the parameter collection holds that argument, which is your set of sums.

Inner loop

Within the reduce loop, each key/value pair of the record is iterated over using forEach. To get the key/value pair the Object.entries method is used. Using array destructuring these arguments can be directly assigned to the respective variables, key and value

Retrieving/Setting values

Unlike a primitive object, Map has its own methods for getting and setting its entries using get() and set(). So first retrieve the previous sum using get(), if it's not set then default to 0, which is what the || 0 does. At that point, you can assume the previous sum is at least 0 or greater and add the current key's value onto it.

Alternatives to Map

If you find Map is a bit heavy-handed, you may also use a similar object such as Set, which has many of the same methods (except the get()), or you could also use a primitive object (i.e. {}).