TypeScript Will Not Let Me Set Value of Object in Array

Question

Why will TypeScript not let me set the value of an object inside an array that I have the exact location of?

list: AdminDefinitionType[];
activeIndex: number;

async updateDefinition(id: keyof AdminDefinitionType, value: string | number | boolean) {
  if (value === this.list[this.activeIndex][id]) {
    return true; // No Change
  } else {
    this.list[this.activeIndex][id] = value;  // 'string | number | boolean' is not assignable to type 'never' - WTF???
    const definition: AdminDefinitionType = await clerk.update(this.list[this.activeIndex])
  }
},

I feel like I have tried everything. I thought it might be because it may be thinking active index could be out of bounds, but I wrapped it in an if to ensure it was and still no luck.

How can I fix this error?

Update

export type AdminDefinitionType = {
  id?: number;
  name: string;
  sort: number;
  disabled: boolean;
};

This is my admin definition type. There are quite a few more props that don't need to be posted here, but they are all typed as strings and not optional.

How is AdminDefinitionType defined ?

Matthieu Riegler
– Matthieu Riegler

2022-08-11 18:49:17 +00:00
Commented Aug 11, 2022 at 18:49 — Matthieu Riegler
– Matthieu Riegler, Commented Aug 11, 2022 at 18:49

Matthieu Riegler · Accepted Answer · 2022-08-11 19:20:18Z

3

You need to specify the type for the key. You can do this with a generic

  async updateDefinition<K extends keyof AdminDefinitionType>(id: K, value: AdminDefinitionType[K]) {
    if (value === this.list[this.activeIndex][id]) {
      return true; // No Change
    } else {
      this.list[this.activeIndex][id] = value; // OK
      const definition: AdminDefinitionType = await clerk.update(this.list[this.activeIndex])
    }
  }

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answered Aug 11, 2022 at 19:20

Matthieu Riegler

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2 Comments

user10012 Over a year ago

You The Man!!! - I would love to better understand why this works though. Can you point me towards a resource I can read?

Matthieu Riegler Over a year ago

The compiler needs to ensure the value you are passing is the right one for the key. That's what the generic is doing. More on generics here : typescriptlang.org/docs/handbook/2/generics.html

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