What is the purpose of importing Vue in my .d.ts file?

Question

What is the purpose of importing Vue in my .d.ts file?

I am confused about the necessity of importing Vue in my .d.ts file. The official website mentions to "Make sure to import 'vue' before declaring augmented types", but I don't understand why this is required. https://v2.vuejs.org/v2/guide/typescript.html#Augmenting-Types-for-Use-with-Plugins

There have been instances where I did not import Vue in my .d.ts files, yet no errors were reported. Despite looking through various articles, I have still not found a satisfactory answer.

typescript vue.js

Answer 1

Answer №1

Vue wants you to merge the class and interface together, essentially extending the Vue class with your own properties.

For instance, consider this example:

class Animal {
  legs: number;
}

interface Animal {
  tail: boolean;
}

Upon creating a new instance of this Animal object, it will have both legs and tail properties.

const animal = new Animal();
animal.legs = 4; // no type error
animal.tail = true; // no type error

However, things get complicated when you attempt to add a method to the interface Animal, as interfaces are meant for structure and not implementation.

To address this issue, TypeScript provides type augmentation, as mentioned in the linked article.

By importing the Animal class and declaring a module with the same name, you can combine the two objects:

import Animal from "./animal";

declare module "./animal" {
  // Here, you can now add the interface
  interface Animal {
    tail: boolean;
    eat(food: string): void;
  }
}

While you still can't implement methods directly in an interface, exposing the object's prototype allows you to include implementation details that were previously inaccessible.

Animal.prototype.eat = function(food) {
  // consume `food` and perform action
}

If you didn't encounter an error without the import statement, it's because TypeScript recognized a module named Vue, and until you access something specific to that import, no error is triggered.

Answer 2

Vue wants you to merge the class and interface together, essentially extending the Vue class with your own properties.

For instance, consider this example:

class Animal {
  legs: number;
}

interface Animal {
  tail: boolean;
}

Upon creating a new instance of this Animal object, it will have both legs and tail properties.

const animal = new Animal();
animal.legs = 4; // no type error
animal.tail = true; // no type error

However, things get complicated when you attempt to add a method to the interface Animal, as interfaces are meant for structure and not implementation.

To address this issue, TypeScript provides type augmentation, as mentioned in the linked article.

By importing the Animal class and declaring a module with the same name, you can combine the two objects:

import Animal from "./animal";

declare module "./animal" {
  // Here, you can now add the interface
  interface Animal {
    tail: boolean;
    eat(food: string): void;
  }
}

While you still can't implement methods directly in an interface, exposing the object's prototype allows you to include implementation details that were previously inaccessible.

Animal.prototype.eat = function(food) {
  // consume `food` and perform action
}

If you didn't encounter an error without the import statement, it's because TypeScript recognized a module named Vue, and until you access something specific to that import, no error is triggered.

What is the purpose of importing Vue in my .d.ts file?

Answer №1

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