Ensuring TypeScript recognizes a class property as definitively initialized when set using a Promise constructor

Question

Ensuring TypeScript recognizes a class property as definitively initialized when set using a Promise constructor

I have a simple class definition that is giving me an error in TypeScript.

class Container {
    resolveData: (s: string) => void // not definitely initialized error!
    data: Promise<string>
    constructor() {
        this.data = new Promise<string>(
            r => this.resolveData = r
        )
    }
}

In this code, TypeScript is complaining about the resolveData property not being definitely initialized.

It seems like TypeScript may not be recognizing that the callback function is synchronously executed. How can I inform TypeScript that this property is indeed initialized?

typescript

Answer 1

Answer №1

To ensure TypeScript recognizes that a property is definitely initialized, you can simply add an exclamation mark ! next to the property definition.

class Container {
    resolveData!: (s: string) => void 
    data: Promise<string>
    constructor() {
        this.data = new Promise<string>(
            r => this.resolveData = r
        )
    }
}

This type assertion serves as a way to inform TypeScript that you have more knowledge about the initialization process than it does, essentially bypassing its checking in that specific case. If used correctly, like in your example, it can be quite helpful. However, if misused and applied when the property is not actually initialized correctly, TypeScript won't be able to catch the mistake for you.

If you want to explore further information on this syntax, it's commonly known as the "Definite Assignment Assertion."

Answer 2

To ensure TypeScript recognizes that a property is definitely initialized, you can simply add an exclamation mark ! next to the property definition.

class Container {
    resolveData!: (s: string) => void 
    data: Promise<string>
    constructor() {
        this.data = new Promise<string>(
            r => this.resolveData = r
        )
    }
}

This type assertion serves as a way to inform TypeScript that you have more knowledge about the initialization process than it does, essentially bypassing its checking in that specific case. If used correctly, like in your example, it can be quite helpful. However, if misused and applied when the property is not actually initialized correctly, TypeScript won't be able to catch the mistake for you.

If you want to explore further information on this syntax, it's commonly known as the "Definite Assignment Assertion."

Ensuring TypeScript recognizes a class property as definitively initialized when set using a Promise constructor

Answer №1

Similar questions

The ngAfterViewChecked function seems to be caught in an endless loop

Trouble with a third-party library component not functioning properly on the server side in a Next.js environment

Tips for successfully importing $lib in SvelteKit without encountering any TypeScript errors

What is the best way to retain all checkbox selections from a form upon submission?

Issue with importing MomentJS globally in TypeScript

Utilizing TypeScript for enhanced Chrome notifications

Tips for sorting an array with various data types in TypeScript while explicitly defining the type

How can I implement a feature in Angular where clicking the edit button loads a form (in a separate component) pre-populated with previous data, along with an update button for

Angular: Real-time monitoring of changes in the attribute value of a modal dialog and applying or removing a class to the element

Display the initial x list items without utilizing ngFor in Angular 2

What is the reason behind Typescript not narrowing generic union type components when they are eliminated by type guards?

"An issue has been identified where TSLint and VSCode fail to display red underlines in

Prisma DB is a versatile database that excels in handling m-n

When trying to pass 3 parameters from an Angular frontend to a C# MVC backend, I noticed that the server side was receiving null

Is it possible for a property to be null or undefined on class instances?

Enable Intellisense for my custom ES6 JavaScript modules in VS Code

Ways to avoid using a specific type in TypeScript

What is the most efficient way to use map-reduce in TypeScript to filter a list based on the maximum value of an attribute?

Build for Node.js using TypeScript with full functionality

The creation of a Firebase user account with the specified email and password