2nd parameter of function depends on 1st

[fringe wolf]

Hi, I am trying to get proper type for 2nd param in a function whose type should depend upon the first one, here is an example

https://www.typescriptlang.org/play?#code/C4TwDgpgBAgmYBsIHkzAJYHsB2BnKAvFAN4BQUU6ATjgFxTYCuAtgEYRWkC+ppokUZFQCG2AOYo0WPIRLkoAN3TBhzdNgDC9Jmw7deAM0bYAxhhxRcwkAAkICBJgAUBqo2X0A5MPhJPUAB8oT0wRcQhPABooV3dgVHM8elEQAEoyClwAd2UTAAsXN2V0+QoTYVxob18I2lKKKBMcXEwkADpHMScAAx9EaDyKqAASYljlBOlcNuocLkoabG7U+opWKghhAGt68srg0NEJTzqGhqa8VogOzC7uoSOBodHx+KlmtqUVNU15r9V1FANMt5DwuEA

[odd edge]

@fringe wolf Here's a shortened URL of your playground link! You can remove the full link from your message.

bhavyacodes#0

Playground Link

Preview:```ts
type AppleOptions = {
iron: number
}

type OrangeOptions = {
vitaminC: number
}

function sayHello(
fruit: "apple" | "orange",
fruitOptions: any
) {
switch (fruit) {
case "apple":
console.log(apple has ${fruitOptions} iron)
break
case "orange":
console.l
...```

You can choose specific lines to embed by selecting them before copying the link.

[fringe wolf]

!ts

[odd edge]

type AppleOptions = {
  iron: number
}

type OrangeOptions = {
  vitaminC: number
}

function sayHello(fruit: 'apple' | 'orange', fruitOptions: any){
  switch(fruit){
    case 'apple':
      console.log(`apple has ${fruitOptions} iron`)
      break
    case 'orange':
      console.log(`Orange has ${fruitOptions.vitaminC} vitamin C`)
  }
}```

[sleek shoal]

is fruitOptions supposed to be the number or an object?

[fringe wolf]

yes its a mistake, im unable to edit

type AppleOptions = {
  iron: number
}

type OrangeOptions = {
  vitaminC: number
}

function sayHello(fruit: 'apple' | 'orange', fruitOptions: any){
  switch(fruit){
    case 'apple':
      console.log(`apple has ${fruitOptions.iron} iron`)
      break
    case 'orange':
      console.log(`Orange has ${fruitOptions.vitaminC} vitamin C`)
  }
}

[sleek shoal]

this is kinda hard to do safely

you could use a generic function, but then the generic could be a union.

it'd be easier if you could form the arguments as a discriminated union

[fringe wolf]

earlier I tried using something like

sayHello(...args: ['apple' , {iron: number}] | ['orange' , {vitaminC: number}]){}

but that didn't work for some reason

[sleek shoal]

oh, right, that is a discriminated union.

[nocturne lion]

Do it with spread parameters + union.

[odd edge]

nonspicyburrito#0

Playground Link

Preview:```ts
type AppleOptions = {
iron: number
}

type OrangeOptions = {
vitaminC: number
}

function sayHello(
...[fruit, options]:
| [fruit: "apple", options: AppleOptions]
| [fruit: "orange", options: OrangeOptions]
) {
swit
...```

You can choose specific lines to embed by selecting them before copying the link.

[sleek shoal]

you need commas between elements, not unions

[fringe wolf]

this worked, tysm 😽

!close

!open

ok it works, but is there a better way, thing is im building a library that has to be used by others, and the error looks like this

if there was a better way using generic please help

[sleek shoal]

what about an object?

[fringe wolf]

hmmmm yeah but this what my boss wants

and i like I would also like it this way, first param to be a string and 2nd depending on the first arg

possible to do something like this?

  async dial<ChainType extends ChainNetwork>(
    chain: ChainType,
    options: ChainType === 'moi' ? MoiOptions : EthereumOptions,
  ) {}

type ChainNetwork = 'moi' | 'ethereum';

[sleek shoal]

conditionals in signatures are almost never what you need

[fringe wolf]

I mean I will be adding the runtime check also manually

[sleek shoal]

also, you could pass ChainNetwork and then pass "moi" and EtheriumOptions

[fringe wolf]

its just for types

[fringe wolf]

I didn't get this, the 2nd part

[nocturne lion]

You can use generic for this, but inside the implementation it would get very ugly.

DU is generally the way to go.

[fringe wolf]

pls show how with the fruit example

pl0x

😽

[odd edge]

nonspicyburrito#0

Playground Link

Preview:```ts
type AppleOptions = {
iron: number
}

type OrangeOptions = {
vitaminC: number
}

type ParameterMap = {
apple: AppleOptions
orange: OrangeOptions
}

function sayHello<T extends keyof ParameterMap>(
fruit: T,
options: P
...```

You can choose specific lines to embed by selecting them before copying the link.

[nocturne lion]

You have no way to fix the error inside your implemenetation and just have to cast.

And it's also unsafe, people can do:

sayHello<'apple' | 'orange'>('apple', { vitaminC: 42 })

And it will explode at runtime.

[fringe wolf]

I think I know what to do

first param is string

I can make an object with those strings as keys

and do some keyof magic with generics

[nocturne lion]

That's what I just gave you.

[fringe wolf]

ohhh lol

[nocturne lion]

But also read my comments following it.

It's not a good solution.

[fringe wolf]

okay, i'll try a little more digging

Thanks!