Specifying the type of "this" within the function body

Question

Specifying the type of "this" within the function body

After going through the typescript documentation, I came across an example that explains how to use type with this in a callback function. I am hoping someone can assist me in comprehending how this callback will operate.

interface DB {
  filterUsers(filter: (this: User) => boolean): User[];
}
 
const db = getDB();
const admins = db.filterUsers(function (this: User) {
  return this.admin;
});

typescript

Answer 1

Answer №1

When it comes to traditional functions and methods, this acts as a read-only parameter passed to the function by the calling code. You can find more information on this topic here. Essentially, when the function is executed, this refers to whatever the caller has specified explicitly or implicitly.

In TypeScript, using the this pseudo-parameter allows you to specify the type that this will have when the function is invoked. For example, in the DB interface, the filterUsers method specifies that this will be a reference to a User object during the callback.

You don't need to restate this type information in your callback function because TypeScript can infer it from the DB interface's declaration:

const db = getDB();
const admins = db.filterUsers(function() {
  return this.admin;
});

In one of the comments, you asked:

How does the function become bound to a specific User instance here?

The logic inside the filterUsers method ensures that when the callback is called, it sets the context (this) to a user object. This might involve using Function.prototype.call or Function.prototype.apply:

// Hypothetical snippet for a `filterUsers` method:
filterUsers(callback: (this: User) => boolean): User[] {
    const filtered = this.users.filter(user => callback.call(user));
    return filtered;
}

Using .call or .apply allows you to explicitly define the context for a function call.

It's worth mentioning that defining DB's filterUsers method in this way is not typically how interfaces would handle such behavior. It would be more conventional for the user to be passed as a regular argument, similar to how Array.prototype.filter works. However, this unconventional approach is used in documentation examples for explanatory purposes.

Answer 2

When it comes to traditional functions and methods, this acts as a read-only parameter passed to the function by the calling code. You can find more information on this topic here. Essentially, when the function is executed, this refers to whatever the caller has specified explicitly or implicitly.

In TypeScript, using the this pseudo-parameter allows you to specify the type that this will have when the function is invoked. For example, in the DB interface, the filterUsers method specifies that this will be a reference to a User object during the callback.

You don't need to restate this type information in your callback function because TypeScript can infer it from the DB interface's declaration:

const db = getDB();
const admins = db.filterUsers(function() {
  return this.admin;
});

In one of the comments, you asked:

How does the function become bound to a specific User instance here?

The logic inside the filterUsers method ensures that when the callback is called, it sets the context (this) to a user object. This might involve using Function.prototype.call or Function.prototype.apply:

// Hypothetical snippet for a `filterUsers` method:
filterUsers(callback: (this: User) => boolean): User[] {
    const filtered = this.users.filter(user => callback.call(user));
    return filtered;
}

Using .call or .apply allows you to explicitly define the context for a function call.

It's worth mentioning that defining DB's filterUsers method in this way is not typically how interfaces would handle such behavior. It would be more conventional for the user to be passed as a regular argument, similar to how Array.prototype.filter works. However, this unconventional approach is used in documentation examples for explanatory purposes.

Specifying the type of "this" within the function body

Answer №1

Similar questions

Making a quick stop in Istanbul and NYC to collect a few important files

Error message: Unable to locate Bootstrap call in standalone Angular project after executing 'ng add @angular/pwa' command

"The ion-label in Ionic2 is cutting off some of the text and not displaying it

summing 3 numbers to a total of 100 percent

In ReactJS, the way to submit a form using OnChange is by utilizing the

In Typescript, is it possible to utilize the value from a particular key in a type declaration as the type for another key within the same declaration?

Strategies for preventing multi-level inheritance of TypeScript class properties and methods

Utilizing data from the home component in another component: A guide

Error: The function call does not match any of the overloads. 'VueRouter' is not recognized

ngPrime table column selection and data extraction

Oops! An issue occurred during the `ng build` command, indicating a ReferenceError with the message "Buffer is not defined

What is the best way to allow the browser to either download a file or open it in a new tab based on what is feasible? (while maintaining the actual file

Having difficulty in utilizing localStorage to update the state

What steps can be taken to initiate Nx Release on the apps/* when modifications are made to the connected libs/* modules?

Dynamic rows in an Angular 2 Material data table

The modal popup feature is dysfunctional within the hierarchical component structure of angular-bootstrap-md

Challenge with Typescript Interfaces

Guide on dividing a URL string in Angular framework

How can I set the default bindLabel for a dropdown in @ng-select/ng-select when the self change event occurs in Angular

Implementing Batch File Uploads using Typescript