CODEWITHSUNDEEP
typescript
be-codified
be-codified

Reputation: 6124

TypeScript error: Type 'void' is not assignable to type 'boolean'

I am having a TypeScript error:

Argument of type '(element: Conversation) => void' is not assignable to parameter of type '(value: Conversations, index: number, obj: Conversation[]) => boolean'. Type 'void' is not assignable to type 'boolean'.

This is my schema:

export class Conversation {
  constructor(
    public id: number,
    public dateTime: Date,
    public image: string,
    public isUnread: boolean,
    public title: string
  ) {}
}

And this is my code

// Mark as read also in data store
this.dataStore.data.find((element) => {
  if (element.id === conversationId) {
    element.isUnread = true;
    // Push the updated list of conversations into the observable stream
    this.observer.next(this.dataStore.data);
  }
});

What does this error mean?

Upvotes: 42

Views: 261969

Answers (2)

John Weisz
John Weisz

Reputation: 31924

It means that the callback function you passed to this.dataStore.data.find should return a boolean and have 3 parameters, two of which can be optional:

  • value: Conversations
  • index: number
  • obj: Conversation[]

However, your callback function does not return anything (returns void). You should pass a callback function with the correct return value:

this.dataStore.data.find((element, index, obj) => {
    // ...

    return true; // or false
});

or:

this.dataStore.data.find(element => {
    // ...

    return true; // or false
});

Reason why it's this way: the function you pass to the find method is called a predicate. The predicate here defines a boolean outcome based on conditions defined in the function itself, so that the find method can determine which value to find.

In practice, this means that the predicate is called for each item in data, and the first item in data for which your predicate returns true is the value returned by find.

Upvotes: 51

NYCdotNet
NYCdotNet

Reputation: 4647

Your code is passing a function as an argument to find. That function takes an element argument (of type Conversation) and returns void (meaning there is no return value). TypeScript describes this as (element: Conversation) => void'

What TypeScript is saying is that the find function doesn't expect to receive a function that takes a Conversation and returns void. It expects a function that takes a Conversations, a number and a Conversation array, and that this function should return a boolean.

So bottom line is that you either need to change your code to pass in the values to find correctly, or else you need to provide an overload to the definition of find in your definition file that accepts a Conversation and returns void.

Upvotes: 11

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