Tips for resolving the TypeScript error "Type 'string' is not compatible with type '\"AOI\" | \"DIMM\" | \"FAN\" | undefined'"

Currently working on a Vue3 project with TypeScript and encountering an error that is puzzling me: https://i.sstatic.net/EWKjy.png

There are 3 possible values being received from the backend, and my kind prop expects one of these strings.

The prop definition looks like this:

kind: { type: String as PropType<'AOI' | 'DIMM' | 'FAN'> }

I'm unsure why this error is occurring, especially since only one of the 3 specified values will be sent from the backend. Any insights on this matter would be greatly appreciated.

If more context is needed, please do not hesitate to ask!

Cheers!

Answer №1

To streamline the process, you can consolidate the strings into a type (as highlighted by @Alexey in their insightful response). This type can then be exported from the component to allow users to define their kind bindings:

<!-- MyComponent.vue -->
<script lang="ts">
import { defineComponent, type PropType } from 'vue';

export type KindType = 'AOI' | 'DIMM' | 'FAN'

export default defineComponent({
  props: {
    kind: { type: String as PropType<KindType> }
  },
})
</script>

Users of this component can utilize type assertion (ref('myString' as KindType)) to instruct TypeScript to interpret any string as a member of KindType, as shown below. It's worth noting that this approach permits incorrect strings (i.e., those outside the defined KindType values), potentially leading to unexpected outcomes.

<!-- App.vue -->
<script setup lang="ts">
import { ref } from 'vue'
import MyComponent, { type KindType } from './components/MyComponent.vue'

const kind = ref('foo' as KindType) // ⛔️ compiles but accepts any string
</script>

<template>
  <MyComponent :kind="kind" />
</template>

A more effective solution involves supplying a type argument to ref, such as ref<KindType>('myString'). By doing so, the compiler will issue an error if the provided string is not valid.

<script setup lang="ts">
import { ref } from 'vue'
import MyComponent, { type KindType } from './components/MyComponent.vue'

const kind = ref<KindType>('FAN') // ✅ restricts input to KindType strings only
</script>

<template>
  <MyComponent :kind="kind" />
</template>

View a live demonstration here.

Answer №2

Is there a way to address this issue directly in the frontend?

Just to clarify, I'm not proposing any modifications to the backend itself but rather suggesting adjustments to where its signatures are specified in the frontend.

If you prefer not to make changes in that regard, you could simply include a type assertion

someBackendMethod(...) as "AOI" | "DIMM | "FAN"
. Essentially indicating "I acknowledge that this specific string falls into one of these 3 categories even if the compiler doesn't recognize it". However, the downside is that if the backend introduces another potential value, the compiler won't flag this location for updating.

Another recommendation is to assign a name to this union:

type SomeName = "AOI" | "DIMM | "FAN"

and subsequently

kind: { type: String as PropType<SomeName> }

someBackendMethod(...) as SomeName

so that in case values are added or removed, only one modification is required.

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