Incorrect value for `this` is displayed in TypeScript by Visual Studio

Question

Incorrect value for `this` is displayed in TypeScript by Visual Studio

Take a look at the code snippet below:

class Person{
    firstname = ko.observable<string>();
    lastname: ko.observable<string>();
    fullname = ko.computed(()=>{

        // Breakpoint here
        return this.firstname() + ' ' + this.lastname();

    });

During debugging in Visual Studio 2013, I noticed that when I set a breakpoint and examine the value of this using watch or immediate window, it displays as window instead of an instance of the person object. As a result, this.firstname returns undefined.

Upon reviewing the generated JavaScript code, I realized that I should be checking the value of _this instead of this.

Even though the code executes without errors, it took me some time to realize that the actual value of this is accessible through _this.

Question: Is there a mistake in how I am using class properties that leads to this discrepancy in the value of this? Or is it possibly a bug? Could it be intentional for certain reasons?

javascript visual-studio visual-studio-2013 typescript typescript1.4

Answer 1

Answer №1

In javascript, Typescript automatically creates an "_this" alias for the "this" keyword. Understanding how this works is crucial in utilizing it effectively.

For example:

class Person {
    firstname = ko.observable<string>();
    lastname: ko.observable<string>();
    fullname = ko.computed(
        () => {
            // Breakpoint here
            return this.firstname() + ' ' + this.lastname();
    });
}

When compiled, it looks like this:

var Person = (function () {
    function Person() {
        var _this = this;
        this.firstname = ko.observable();
        this.lastname = ();
        this.fullname = ko.computed(function () {
            // Breakpoint here
            return _this.firstname() + ' ' + _this.lastname();
        });
    }
    return Person;
})();

This shows that "this" inside the fullname computed function has been replaced with "_this" during compilation. Debugging issues may arise because Visual Studio debugs the compiled javascript where "this" behaves differently. Learn more about "this" in javascript here.

Typescript introduces a _this reference when using lambda functions, such as:

class foo {

    something: string = "some string";

    foo1 = () => { this.something }
    foo2() { this.something }
}

Which compiles to:

var foo = (function () {
    function foo() {
        var _this = this;
        this.something = "some string";
        this.foo1 = function () { _this.something; };
    }
    foo.prototype.foo2 = function () { this.something; };
    return foo;
})();

Lambdas in Typescript help avoid the "this" confusion commonly found in javascript. While you usually don't need to differentiate between lambdas and functions, there are cases where it matters. Explore more about lambda functions here.

To handle "this" effectively, pay attention to _this when working with lambdas until the issue is resolved. An open issue regarding this matter can be found at: .

Answer 2

In javascript, Typescript automatically creates an "_this" alias for the "this" keyword. Understanding how this works is crucial in utilizing it effectively.

For example:

class Person {
    firstname = ko.observable<string>();
    lastname: ko.observable<string>();
    fullname = ko.computed(
        () => {
            // Breakpoint here
            return this.firstname() + ' ' + this.lastname();
    });
}

When compiled, it looks like this:

var Person = (function () {
    function Person() {
        var _this = this;
        this.firstname = ko.observable();
        this.lastname = ();
        this.fullname = ko.computed(function () {
            // Breakpoint here
            return _this.firstname() + ' ' + _this.lastname();
        });
    }
    return Person;
})();

This shows that "this" inside the fullname computed function has been replaced with "_this" during compilation. Debugging issues may arise because Visual Studio debugs the compiled javascript where "this" behaves differently. Learn more about "this" in javascript here.

Typescript introduces a _this reference when using lambda functions, such as:

class foo {

    something: string = "some string";

    foo1 = () => { this.something }
    foo2() { this.something }
}

Which compiles to:

var foo = (function () {
    function foo() {
        var _this = this;
        this.something = "some string";
        this.foo1 = function () { _this.something; };
    }
    foo.prototype.foo2 = function () { this.something; };
    return foo;
})();

Lambdas in Typescript help avoid the "this" confusion commonly found in javascript. While you usually don't need to differentiate between lambdas and functions, there are cases where it matters. Explore more about lambda functions here.

To handle "this" effectively, pay attention to _this when working with lambdas until the issue is resolved. An open issue regarding this matter can be found at: .

Answer 3

Answer №2

Discovering the intricacies of JavaScript can sometimes feel like navigating a maze. The concept of "this" can be particularly bewildering, leaving developers scratching their heads and muttering, "What is this referring to?" But fear not, for this behavior is not a glitch but a deliberate design choice. JavaScript dynamically reassigns the value of "this" based on the execution context, leading to potential confusion, especially when working with libraries like d3, angular, or knockout.

The challenges posed by dynamic scoping are magnified in TypeScript, where explicit references to "this" are required throughout the codebase. To gain a better understanding of how "this" works, consult the detailed documentation available on the Mozilla Developer Network.

To circumvent these complexities, a simple yet effective solution is to store a reference to the original "this" before entering your callback function. By using a local variable within the function, you can sidestep potential issues:

class Person {
  firstname = ko.observable<string>();
  lastname: ko.observable<string>();
 var self = this;
 fullname = ko.computed(() => {

    // Insert a breakpoint here
    return self.firstname() + ' ' + self.lastname();

  });

Developing this habit can help prevent future headaches and streamline your JavaScript coding practices. Remember, mastering the nuances of "this" is key to becoming a more proficient JavaScript developer.

Answer 4

Discovering the intricacies of JavaScript can sometimes feel like navigating a maze. The concept of "this" can be particularly bewildering, leaving developers scratching their heads and muttering, "What is this referring to?" But fear not, for this behavior is not a glitch but a deliberate design choice. JavaScript dynamically reassigns the value of "this" based on the execution context, leading to potential confusion, especially when working with libraries like d3, angular, or knockout.

The challenges posed by dynamic scoping are magnified in TypeScript, where explicit references to "this" are required throughout the codebase. To gain a better understanding of how "this" works, consult the detailed documentation available on the Mozilla Developer Network.

To circumvent these complexities, a simple yet effective solution is to store a reference to the original "this" before entering your callback function. By using a local variable within the function, you can sidestep potential issues:

class Person {
  firstname = ko.observable<string>();
  lastname: ko.observable<string>();
 var self = this;
 fullname = ko.computed(() => {

    // Insert a breakpoint here
    return self.firstname() + ' ' + self.lastname();

  });

Developing this habit can help prevent future headaches and streamline your JavaScript coding practices. Remember, mastering the nuances of "this" is key to becoming a more proficient JavaScript developer.

Incorrect value for `this` is displayed in TypeScript by Visual Studio

Answer №1

Answer №2

Similar questions

What is the best way to iterate through a designated key in every object within a JSON dataset and store it in a variable called `data` for use with

Utilizing Express Routes to specify React page components

Joi integration for Multer validation

Mapping an array of objects within another array of objects

Troubleshooting compatibility issues between ExpressJS's res.render and AngularJS

When HTML elements are dynamically inserted through JavaScript using quilljs, they may cause conflicts with the layout properties

Order the rows being inserted into a table by iterating through them using a foreach loop directly from the

Setting individual state variables for each child component within a Vue loop

Tips for accurately assigning types when updating state in React

Encountering an unexpected token ';' in a random generator while attempting to utilize an if-else statement for determining DOM manipulation

Tips for uploading a file with fetch

Tips for managing a dblclick event on a row with data in a kendo-grid while using Angular2

Unable to integrate npm package into Nuxt.js, encountering issues with [vue-star-rating] plugin

Utilize AJAX to compare an inputted location with geocodes stored in an SQL database

Discord bot in Node.js, Remove old embed message upon receiving a slash command

Submit the form to MailChimp and then show a personalized success page

Prevent the use of harmful language by implementing jQuery or JavaScript

What is the best way to target and manipulate the transform property of multiple div elements in JavaScript?

Caution: An invalid next.config.js file has been detected while running the Next.js project

An issue has occurred: TypeError - It is impossible to access the 'forEach' property of an undefined object