---
title: 'Angular2: method callback type in TypeScript'
url: https://blog.octoperf.com/angular2-method-callback-type-in-typescript/
date: '2016-04-03'
authors:
- gpereira
categories:
- Development
tags:
- Angular
- Typescript
description: How to set the type for a method callback in TypeScript when using Angular2 Http service.
---

# Angular2: method callback type in TypeScript

It's my third post about Angular2 and TypeScript as I'm creating an admin console for [OctoPerf load testing platform](https://octoperf.com).

## Intro

This article explains **how to set a type for callbacks**. If, like me, you are used to AngularJS you may often use callbacks for asynchronous calls, typically for HTTP requests.

I wrote a *StripeClient* service that can return the list of my Customers. In pure JavaScript it would be fairly simple as you don't need to give a type to your callback.

## In Angular2 / TypeScript

In TypeScript it's a bit more complicated, but cleaner.


First I need to define my customer:
```javascript
export class Customer{
  constructor(public email: string, public id:string) {
  }
}
```

Then the method *getCustomers* of the *StripeClient* service: 
```javascript
getCustomers(callback: (customers: Customer[])=>void) {
    this.http.get(stripeUrl + '/customers', {headers: this.getAuthHeaders()})
      .map(res => res.json())
      .subscribe(paginatedList => callback(<Customer[]>paginatedList.data));
  }
```

This method uses the *Http* Service to call the Stripe API.
It has only one parameter: the callback function.

**The type for the callback is `(customers: Customer[])=>void` a method that takes a Customer array as a parameter and returns nothing.**
I don't know why be we have to give a name to our customers array.  
 
This method simply calls its callback with the customers returned by the API call. 
We need to cast the result when doing so: `callback(<Customer[]>paginatedList.data)`.

Stripe returns paginated lists, and we need to make successive calls to their API to get all our customers, plans, etc. 
It's a good scenario to learn more about [generics in TypeScript](https://blog.octoperf.com/angular2-using-typescript-generics/).