Checking Types Against the Real World in TypeScript

This is an excerpt of a post that I wrote for Olio Apps.

The shape of data defines a program. There are important benefits to writing out types for your data.

Let’s consider a Hacker News client, which consumes stories and other items from the Hacker News API. This is a TypeScript type that describes the format for stories:

type Story = {
  type: "story",
  by: string,  // username
  dead?: boolean,
  deleted?: boolean,
  descendants: number,
  id: number,
  kids?: number[],  // numeric IDs of comments
  score: number,
  text?: string,  // HTML content if the story is a text post
  time: number,  // seconds since Unix epoch
  title: string,
  url?: string  // URL of linked article if the story is not a text post
};

In Javascript and other dynamically-typed languages, it is common to write a program without any explicit description of a data structure like Story. The shape of the data is implied in the code that manipulates the data. But that means anyone reading the code has to mentally reconstruct that shape from context, or refer to documentation outside of the program itself.

» Read the rest on the Olio Apps blog.