Pfthb

I'm fairly familiar with Javascript, but having some issues recreating some of the patterns I use in TypeScript, because I'm not sure how to type functions that could have diverse JSON fields. For example, I was trying to write a simple http request function that I could use:

async function requestTest (options: https.RequestOptions) {
 const prom = new Promise((resolve, reject) => {
 https.request(options, (res) => 
 res.on('data', (d) => 
 if (d.status == 200) 
 resolve(d)
 
 reject('failed')
 )
 )
 return await prom

However, if I actually want to use this function, say query a database and get back a result where the "data" field on the response will contain data of a known structure that I have an interface declared for:

const getData = (opts): ISomeStructure => 
 return requestTest(opts)

I get an error of the type:

Type 'Promise<>' is not assignable to type 'ISomeStructure'.

How do I declare the types in such a way as to tell typescript that in this case, I am expecting the http response to have a key that contains an ISomeStructure JSON object?

There are two problems with your code, one related to async code, the other related to types.

The async issue is that you return requestTest(opts) while you declare that your function will return ISomeStructure. Since requestTest retruns a Promise you should return a Promis<ISomeStructure> from getData, since the data returned by requestTest will not be immediately available to return directly.

The second problem is that you should allow the return type of the request to be specified as a type parameter on requestTest that way your function can be used for any returned type.

function requestTest<T>(options: https.RequestOptions) 
 return new Promise<T>((resolve, reject) => 
 https.request(options, (res) => 
 res.on('data', (d) => 
 if (d.status == 200) 
 resolve(d)
 
 reject('failed')
 )
 )
 );


const getData = (opts: https.RequestOptions): Promise<ISomeStructure> => 
 return requestTest<ISomeStructure>(opts)