How do I build an API that lets me do `await t.a().b()` & `await t.a(); await t.b()`?

I want to build an API that lets me chain in two ways:

await t
  .a()
  .b()
  .c()
...
// or
const t = ...;
await t.a();
await t.b();
await t.c();
...

await t
  .a()
  .b()
  .c()
...
// or
const t = ...;
await t.a();
await t.b();
await t.c();
...

This is known as the builder pattern. Usually it's straightforward, but I'm not sure how to create one when promises are involved. NOTE: This is the way https://testcafe.io/ 's api works. I'm trying to build a similar api. See https://discord.com/channels/436251713830125568/1158962809401516062/1159217101471481896 (links to a few messages down)

21 Replies

dys 🐙•14mo ago

You are wanting to call a() and have it return a Promise or an object with a b function deepending on how it was called. (The "dot" (.) operator is higher precedence than await, so await a().b().c() === await (a().b().c())) . There is no way within a function call to know whether the returning context is expecting a Promise or not, so you can't do what you're describing. I mean you could return a Promise from a() that has a b method, but you need to wait for a() to resolve before you'd have a value to do anything with in b, and you're not waiting in a().b().c(). (Note that the then() method of a Promise is called with the result of the Promise & accepts a Promise as output, but that's a special case.) Are you talking about the two code examples on https://testcafe.io? They're two different frameworks: Testcase & Selenium, not different examples of a single framework. If you're not taking about those pieces of code, where are you seeing these alternate syntaxes?

Bawdy Ink Slinger•14mo ago

@dys 🐙 I'm only referring to the left hand side. It doesn't show it being called like in my second example~~, but I know it works both ways because I've got many examples in my project~~ EDIT: Oops. I was wrong. I've updated my OP

There is no way within a function call to know whether the returning context is expecting a Promise or not, so you can't do what you're describing.

I believe the library returns a promise that's been assigned other functions to make it so the code doesn't know how its being called; the single return can handle both scenarios

Jochem•14mo ago

Dysbulic is correct, what you're describing isn't possible. a can either return a promise or an object with other methods, not both. If it returns a promise, chaining won't work without awaiting the promise first. If it returns an object, awaiting it in the second example will do nothing

Rägnar O'ock•14mo ago

You could maybe do it if the b() and c() methods internally call this.then() or await this. But I'm not sure if it would actually work.

Jochem•14mo ago

I don't think you can know whether a method is being called with await in the method itself though

Rägnar O'ock•14mo ago

You don't need to You can call .then() on a promise that has already been fulfilled So you just do it every time the function is called and use the resulting promise/value in the body of the method

Jochem•14mo ago

it feels like a terrible idea overall though, to have a method that returns Schrodinger's Promise

Rägnar O'ock•14mo ago

What do you mean by "Schrodinger's Promise" ? A promise that is maybe already fulfilled?

Jochem•14mo ago

effectively yeah

Rägnar O'ock•14mo ago

'Cause that's what promises are for, doing stuff after some other stuff has finished running.

Jochem•14mo ago

regardless, it makes the await pointless, right? Cause you can call it or not call it (on a() and b()), and the result is the same

Rägnar O'ock•14mo ago

Yes But maybe the results of a() has other stuff they are interested in And actually the first .b() and the second aren't even called on the same object. One's on the promise the other on the value inside the promise so you don't even need to have the same implementation in both. They just need to have the same signature for it to work (a === await a // false) I'll try something when I get home

Bawdy Ink Slinger•14mo ago

a can either return a promise or an object with other methods, not both

    const promise = Promise.resolve().then(() => console.log('then'));
    const both = Object.assign(promise, { myFun: () => console.log('myFun') });
    both.myFun();
    await both;

// myFun
// then

    const promise = Promise.resolve().then(() => console.log('then'));
    const both = Object.assign(promise, { myFun: () => console.log('myFun') });
    both.myFun();
    await both;

// myFun
// then

The above runs. Is this different from what you mean?

Jochem•14mo ago

hm, I guess not. But that feels unbelievably cursed

Bawdy Ink Slinger•14mo ago

agreed. I didn't know it was possible until I tried to copy this API but the API is really great, even if the impl is weird actually let me test something okay I've been mistaken

  const xxx = await t
    .click(Selector('.passage button').withText('a'))
    .expect(Selector('.flash-message').innerText)
    .eql('a')

    await xxx
    .click(Selector('.passage button').withText('b'))
    .expect(Selector('.flash-message').innerText)
    .eql('b')
    .click(Selector('.passage button').withText('c'))
    .expect(Selector('.flash-message').innerText)
    .eql('c')

// TypeError: Cannot read properties of undefined (reading 'click')

  const xxx = await t
    .click(Selector('.passage button').withText('a'))
    .expect(Selector('.flash-message').innerText)
    .eql('a')

    await xxx
    .click(Selector('.passage button').withText('b'))
    .expect(Selector('.flash-message').innerText)
    .eql('b')
    .click(Selector('.passage button').withText('c'))
    .expect(Selector('.flash-message').innerText)
    .eql('c')

// TypeError: Cannot read properties of undefined (reading 'click')

I guess I've never tried the above, I only thought I had. But this works:

  await t
    .click(Selector('.passage button').withText('a'))
    .expect(Selector('.flash-message').innerText)
    .eql('a')

  await t
    .click(Selector('.passage button').withText('b'))
    .expect(Selector('.flash-message').innerText)
    .eql('b')
    .click(Selector('.passage button').withText('c'))
    .expect(Selector('.flash-message').innerText)
    .eql('c')

  await t
    .click(Selector('.passage button').withText('a'))
    .expect(Selector('.flash-message').innerText)
    .eql('a')

  await t
    .click(Selector('.passage button').withText('b'))
    .expect(Selector('.flash-message').innerText)
    .eql('b')
    .click(Selector('.passage button').withText('c'))
    .expect(Selector('.flash-message').innerText)
    .eql('c')

I suppose that's much easier to implement updated my OP. Still not sure how to do this, either

Rägnar O'ock•14mo ago

I'm a bit latte... but I tried to iterate upon this... and it became even more cursed : https://www.typescriptlang.org/play?target=7#code/C4TwDgpgBAcg9sAynAggZxAOwMYB4AqAfFALxQDeAUFDVANoDWUAlplAxCHAGZT4C6ALigAKAJSli6LNgBCAV2DxgBRv0IBuSgF9KoSFGk4FShAWJkACgCc4AW2ZoI5qADJYCZEbxEtlbvI4wMxwbACGGDgA8tbKBFAQAB7AEJgAJmhQAEoQ2HDWabhowNasAOYANFBhmCCEhCIBOMLiklA29o7ORFAAPnxVYYqokdgAshB2AEYQ1mgtHFy8+GJ0-GLC3iZxPVS0UHmYxVAAbmEANvLQZE3Y4lr7h8d2kzNzpFBRUwBWucAAdNxbHYAKKYErMCBoETUfZwmhDYAjGQTaazND-OxhMAiF5o2JhF5tOiw+FkqB4t4wQkQCqk8lwiIyUQSEjEPYMzlPYDVADuYWYKTSADULlcPmF+YLTmKIA9OQzuRTXrMJVKhaLLhA6JTZtSXvw-ArFaE0HBzhB-vzrJgRGctYN1RARbKqrqCS83SrrGJ5cb4fT-bRrBBgPIbcr8X7-bpjetA7RffSAPTJqAAAWAaAAtElINhgLnrLZrAcagByHlYsD0JisdicHh8fhQIH2KDAAAW0GrZfO53pIbDEa+vwL-wiaGYZVtModkbemSTukoSuwEtGMWUIiZOBZbRE5HtV2E5amcCm5e0YiqdHLx4g5fWfiVUw3Mi3CB3o33bNE5AOKBr1vctsCfJNV1NHkwnfaJYi-Xd11aP9DygN9gPoM9wJfU1zUtc44DKHcqimKpsCTJ48P+AiiLCf4pnEQZ6Io3CLWowiRCmf47hvNDuN9IA

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Rägnar O'ock•14mo ago

(don't ask me what it does or how it does it... I don't understand it either as for https://discord.com/channels/436251713830125568/1158962809401516062/1159219191400890428 you have the have your t, t.a() and t.b() either be the same object (by doing return this in every method) or implement the same interface (basically same as before but the object that gets returned is not the same as the one that was called upon)

Rägnar O'ock•14mo ago

(another ugly link : https://www.typescriptlang.org/play?target=7#code/JYOwLgpgTgZghgYwgAgMpgK4xgSXNeJZAbwChkLk4AKASgC41Ns9JZEIBucygIzsbosufOyTcAvqVIIANnADOCpsOTAAtgAdZEdRHDKhLUYRRlKyBAHsQCsFAwIwVqHRJSeFGrRKeLlmwUrHQA6WSsAc2oAcgQ4WR0AEypo2m5-SigITCgQZDAAC2AFdMoPC34fcwyA22CIMMiYuISIZN5U0oysnLzC4q7kKQ9rWzB85ABeZBAIAHcVbDpuGUDxuCn8kO9uAHpdy3ikqlWx5F5NsBDKvYOW495SXm3l0n3D1uS4aSvva9f3vc2idAUdgbwgA)

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The Playground lets you write TypeScript or JavaScript online in a safe and sharable way.

Bawdy Ink Slinger•13mo ago

@Rägnar O'ock thanks! I'll check these out soon

dys 🐙•13mo ago

If you're unaware, Discord supports Markdown links now.

Rägnar O'ock•13mo ago

I am aware... but there's no point in hiding those links

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How do I build an API that lets me do `await t.a().b()` & `await t.a(); await t.b()`?