What exactly occurs within the subscribe function in Angular's Tour of Heroes?

Question

What exactly occurs within the subscribe function in Angular's Tour of Heroes?

I've been stumped trying to figure out the inner workings of the mysterious .subscribe method.

 getHeroes(): void {
    this.heroService.getHeroes()
        .subscribe(heroes => this.heroes = heroes);
  }

Initially, I believed that this.heroes = heroes was assigning values to heroes:Hero[] within the heroes component. However, it appears that this assumption is incorrect. Surprisingly, even if I alter heroes:Hero[] to heroes:boolean, or remove it entirely from the heroes component, my heroes are still displayed and this.heroes is still assigned.

Could someone shed some light on how this is happening?

Check out a live example here

Feel free to experiment with the suggested changes, such as deleting heroes:Hero[], and observe how the heroes remain visible. If this.heroes = heroes isn't assigning to heroes:Hero[], what exactly is occurring within the subscribe method?

javascript angular typescript subscribe

Answer 1

Answer №1

Deleting the heroes property results in the automatic creation of a new property when retrieving data from the service in JavaScript. The browser executes the Javascript code without any errors. However, failure to declare a property or specifying an incorrect datatype will lead to TypeScript compile time error.

Answer 2

Deleting the heroes property results in the automatic creation of a new property when retrieving data from the service in JavaScript. The browser executes the Javascript code without any errors. However, failure to declare a property or specifying an incorrect datatype will lead to TypeScript compile time error.

Answer 3

Answer №2

I used to believe that assigning this.heroes = heroes would update the data in the heroes component

However, the errors highlighted in red are actually TypeScript errors. While TypeScript may overlook them and still compile the project, JavaScript will not have an issue with dynamically creating a new object property.

Nevertheless, a production build should result in a failure.

Answer 4

I used to believe that assigning this.heroes = heroes would update the data in the heroes component

However, the errors highlighted in red are actually TypeScript errors. While TypeScript may overlook them and still compile the project, JavaScript will not have an issue with dynamically creating a new object property.

Nevertheless, a production build should result in a failure.

Answer 5

Answer №3

In this code snippet, the line this.heroes = heroes is crucial as it assigns the received data from the subscriber to the variable 'this.heroes'. It's important to initialize 'this.heroes' as an empty variable at the top of the code to prevent errors when trying to use it later on in the component.

Answer 6

In this code snippet, the line this.heroes = heroes is crucial as it assigns the received data from the subscriber to the variable 'this.heroes'. It's important to initialize 'this.heroes' as an empty variable at the top of the code to prevent errors when trying to use it later on in the component.

Answer 7

Answer №4

Even if you remove the line heroes: Hero[];, it will still appear on the display because TypeScript compiles down to JavaScript in the end.

To illustrate this with a simpler example, imagine adding to an object without first assigning it:

var exampleObj = {};

exampleObj.heroes = 'batman';

console.log(exampleObj);

In JavaScript, this is acceptable and that's why your list still shows up. However, in TypeScript, this approach isn't valid and will result in compilation errors.

Even if I modify heroes:Hero[] to heroes:boolean or delete it entirely from the heroes component, my heroes are displayed and this.heroes is still assigned.

So essentially, they show up because they're added to the class as soon as you define them within the subscription.

Valid in JavaScript, Yes.

Valid in TypeScript, No.

Check out this documentation for more insights into the differences between the two languages.

Answer 8

Even if you remove the line heroes: Hero[];, it will still appear on the display because TypeScript compiles down to JavaScript in the end.

To illustrate this with a simpler example, imagine adding to an object without first assigning it:

var exampleObj = {};

exampleObj.heroes = 'batman';

console.log(exampleObj);

In JavaScript, this is acceptable and that's why your list still shows up. However, in TypeScript, this approach isn't valid and will result in compilation errors.

Even if I modify heroes:Hero[] to heroes:boolean or delete it entirely from the heroes component, my heroes are displayed and this.heroes is still assigned.

So essentially, they show up because they're added to the class as soon as you define them within the subscription.

Valid in JavaScript, Yes.

Valid in TypeScript, No.

Check out this documentation for more insights into the differences between the two languages.

What exactly occurs within the subscribe function in Angular's Tour of Heroes?

Answer №1

Answer №2

Answer №3

Answer №4

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