Dynamically finding TypeScript object values through passed in parameters

Question

not sure if the title is misleading or actually is asking what I want: I am trying to get an Object's element value, but the tricky thing is that the element's name is being passed in through a function...

Here is a simplified sample of my code, hope it's clear what I am trying to do

export interface logItem {
   id: string
   procedure: string
   units: string
   serviceCode: string
}

function calculateTotalsBasedOnType(
   logItems: Array<logItem>
   totalsType: string
): totals {
   ... log.totalsType ...
}

what's the way to go about finding the specific log's element value through the passed in totalsType?

Here is an example method for this

calculateTotalsBasedOnType(logItems, 'serviceCode')

the line log.totalsType should give me the value of the serviceCode from the log

What is the value of `totalsType`? Is it one of `id`, `procedure`, `units`, or `serviceCode`? Also, which element in the array should it get? — Heretic Monkey, Jun 24 '21 at 17:26
the element that it is trying to get is the value of the totalsType: calculateTotalsBasedOnType( log, 'serviceCode') — Stevan Najeeb, Jun 24 '21 at 17:28
There's no way to get "the value of the `serviceCode` from the `log`", since `log` is an array of objects, each with a `serviceCode`. In other words, `log` is something like `[ { id: 1, procedure: 'procedure', units: 12, serviceCode: 'abc' }, { id: 2, procedure: 'erudecorp', units: 21, serviceCode: 'cba' } ]` (I don't know what the actual values are obviously). — Heretic Monkey, Jun 24 '21 at 18:14
If you want to get the property whose name is stored in `totalsType`, that's answered in [Dynamically access object property using variable](/questions/4244896/dynamically-access-object-property-using-variable) — Heretic Monkey, Jun 24 '21 at 18:17
@HereticMonkey yes that's my fault, I did not show that `log.serviceCode` is a single item from that array — Stevan Najeeb, Jun 24 '21 at 18:18
@HereticMonkey i tried that, it still does not liking the `totalsType` being a string. I actually figured it out, I needed to make the `totalsType` to a string literal — Stevan Najeeb, Jun 24 '21 at 18:26

score 0 · Answer 1 · answered Jun 24 '21 at 18:34

In order for typescript to understand that the passed in parameter is a element key of an object you need to make that parameter a string literal type

type TotalTypeValue = 'serviceLine' | 'procedure'

function calculateTotalsBasedOnType(
   logItems: Array<logItem>
   totalsType: TotalTypeValue
)

now when trying to do log.totalsType it will look for the value of totalsType to be one of the elements in the log

score 0 · Answer 2 · answered Jun 24 '21 at 18:54

The question is pretty vague on the subject what exactly you're trying to achieve. But if I got it right, to get the type of the inner field from array you'll have to use keyof operator and lookup types:

function calculateTotalsBasedOnType<T, K extends keyof T>(
   logItems: Array<T>,
   totalsType: K
): T[K] {
   return logItems[0][totalsType] // just random element from the source array
}

playground link

Dynamically finding TypeScript object values through passed in parameters

2 Answers2