just-my-type

Experimental!!!

Attempts to infer a typescript type by inspecting runtime javascript objects.

Why would I want this?

• You have a large API that is not currently typed
• There are no/poor specifications or documentation of what the shape of the objects it returns
• The responses it return vary, and it’s difficult to know that you have a complete understanding of what it can return
• You want to write a type for it

How to use:

The idea is that you want to run this on a real-world system, observing the real-world data for a while, then after some time has passed and you’ve observed many responses, you ask it to suggest a type that will match for all of them.

For each object you’d want to generate a type for, you’d create an instance of TypeGenerator then call observe, passing the data that you’re wanting to generate a type for. After some time has passed and you think you have a complete understanding of what the API does, you’d call suggest and it’ll make you a typescript type that fits the observed data.

Setup (node.js):

npm install just-my-type

import TypeGenerator from 'just-my-type';

Setup (deno):

import TypeGenerator from 'https://deno.land/x/just_my_type@0.0.1/main.ts';

Usage

const tg = new TypeGenerator('MyAPI');

// Assumption: this api is being called by your application somewhere on a regular basis.
export async function myApi() {
  const data = await (await fetch('https://someserver.local/myApiThatINeedATypeFor/')).json();
  tg.observe(data);
  return data;
}
// This is a bad way to do this, but illustrates the point that `observe` should be called multiple times 
// With all the different variations that need to be observed before calling `suggest`
setInterval(() => console.log(tg.suggest()), 5 * 60 * 1000);

Every 5 minutes you’d see something like this in the logs that would be the current representation of all the data that has been observed:

export type MyAPI = {
  startIndex: number,
  maxReturn: number,
  items: Array<{
    type: string,
    children: Array<{
      x: number,
      y: number
    }>
  } | {
    type: string,
    messageText: string
  } | {
    type: string,
    responses: Array<string>
  }>
}