Typed Objects in v-for - Vue3 with TypeScript

I just ran into the following situation in Vue3 and prayed at the altar of the TypeScript gods for an answer. After much beard-pulling and some choice phrases, I found something that works. Is this the way? I cannot confirm nor deny that 😃 but it works and makes sense as well.

Problem

<td v-for="value in currentRecord" :key="recordId">
  {{ value.displayText ? value.displayText : value }}
</td>

Seems innocent enough right? Well, TypeScript had something to say about that, quote: 'value' is of type 'unknown'. ts(18046). My first thought was to type value but there seems to be no way to do this inside the template portion of a single file component.

So where does currentRecord come from? Over here:

const currentRecord = Object.fromEntries(
  Object.entries(props.record).filter(([key]) => key !== "id")
);

The Solution

If I typed currentRecord, would TypeScript then be happy inside the template? Yes it will, but there is another problem here. The currentRecord object is a bit tricky in that it can contain properties that have a number, or string as the value and some can be an object itself. Something like this:

{
  id: 1,
  word: "Monkey",
  phoneme: "i",
  blendDirection: {
    id: 1,
    displayText: "reverse",
  },
}

I could create a type that details every property of the object and where relevant indicates that it can be a string or an object like this:

type Record = {
  id: number;
  word: string;
  blendDirection: string | {
    id: number;
    displayText: string;
  };
  // other properties
};

However, there is another way that will be a bit less of a maintenance burden and is then also the type I used in the end:

type Record = {
  [key: string]:
    | string
    | number
    | undefined
    | {
        id: number;
        displayText: string;
      };
};

With this, we have an object that contains a key of type string that can have any of the listed values. Because undefined is one of those, the property itself can be optional. Now let's type currentRecord. Doing it this way will make TypeScript sad:

const currentRecord: Record = Object.fromEntries(
  Object.entries(props.record).filter(([key]) => key !== "id")
);

What you want to do is this:

const currentRecord = Object.fromEntries(
  Object.entries(props.record).filter(([key]) => key !== "id")
) as Record;

OK, now everything should be good, right?

Wrongo!

The first thing that TypeScript is going to complain about is that value can be undefined. I get why it would say that we told TypeScript that that could be the case. So this should do the trick:

{{ value?.displayText ? value.displayText : value }}

Big Nope!

Because the value could be a string or a number, TypeScript will say, Property 'displayText' does not exist on type 'string | number | { id: number; displayText: string; }'.
Property 'displayText' does not exist on type 'string'.ts(2339)

Fair enough 🥺 - So what to do... The final piece of the puzzle is to check whether value is an object and only then grab displayText, like so:

{{ typeof value === 'object' ? value.displayText : value }}

And that is it! Now, your code works (I did already, but we want to ensure TypeScript is happy as well right?) and TypeScript is happy.

I hope you found this helpful and that it will save you some time and hair. Happy coding!

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