How to sum values in typescript array based on array items property?

Question

I'm working on small angular project. I have an array of receipt items, e.g. coke, fanta, pepsi, juice etc, with their prices and quantity of course.

receiptItems: Array<ReceiptItem>;

This is how ReceiptItem looks :

export class ReceiptItem {

  public id: string;
  public product: Product;
  public unitOfMeasure: UnitOfMeasure;
  public discount: number;
  public price: number;
  public quantity: number;
  public total: number;
  public tax:Tax;

 }

How can I in typescript get sum of total amount but only where property tax for example is equal to "25%"?

In C# I remember I've used lambda expressions like this:

IEnumerable<ReceiptItems> results = receiptItems.Where(s => s.Tax == "25.00");
   totalSum = results.Sum(x => (x.TotalAmount));

How to achieve something similar in TypeScript / Angular?

Suren Srapyan · Accepted Answer · 2018-08-27 12:06:47Z

183

Arrays in JavaScript/TypeScript also have these kind of methods. You can again filter with you condition and then use reduce aggregation function to sum the items.

const sum = receiptItems.filter(item => item.tax === '25.00')
                        .reduce((sum, current) => sum + current.total, 0);

item.tax === '25.00' - this part you must adjust with your logic

edited Aug 27, 2018 at 12:06

answered Aug 27, 2018 at 11:29

Suren Srapyan

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5 Comments

Roxy'Pro Over a year ago

what does this line means? reduce((sum, current) => sum + current.total, 0);

Suren Srapyan Over a year ago

reduce is an aggregation function like Aggregate in C#. It gets an total an every item in the iteration and in this case we sum each item's total to the total (sum) and finally get the sum

Romain Deneau Over a year ago

@Roxy'Pro: there's no direct JavaScript equivalent to Sum() LINQ function. But Sum() can be written using Aggregate().

Romain Deneau Over a year ago

I like splitting operation like you do : filter then sum with reduce.

Romain Deneau Over a year ago

@roxypro: Notice that, as filter produces an intermediate array, in case receiptItems contains a lots of items, the filtering part can be done directly in the reduce : receiptItems.reduce((sum, x) => sum + (x.tax === '25.00' ? x.total : 0));

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