在其静态方法内访问类

Question

I am building a simple class . My question is:

How to access a class method, or this for that matter, inside a static class method?

When trying to access this inside a static method like this:

const { ProductsCollection } = require('../utils/Collection')
let modeledCollection = ProductsCollection.mapToModel(legacy)

I get a TypeError : this.generateModel is not a function . Here is my class:

class ProductsCollection {
    generateModel () {
        let model = { name: 'testing' }
        return model
    }

    static mapToModel (legacy) {
        if (!isObject(legacy))
            return legacy

        let current = this.generateModel() // Here!!!
        for (let key in legacy) {
          // some code...
        }
        return current
    }
}

module.exports = { ProductsCollection }

Thanks in advance!

Answer 1

How to access a class method, or this for that matter, inside a static class method?

The only way to access instance information from a static method is to create an instance (or receive one as a parameter, or close over one [which would be very odd], etc.; eg, you need an instance). This is the point of static methods: They aren't associated with an instance of the class, they're associated with the constructor function.

Your generateModel method, as shown, doesn't use the instance either, so it may make sense for it to be static as well. Then you'd access it via this.generateModel (assuming mapToModel is called via ProductsCollection.mapToModel ) or ProductsCollection.generateModel (if you don't want to make that assumption):

 class ProductsCollection { static generateModel() { return {name: "testing"}; } static mapToModel(legacy) { return this.generateModel(); // or `return ProductsCollection.generateModel();` if you want to use // `ProductsCollection` specifically and not be friendly // to subclasses } } console.log(ProductsCollection.mapToModel({})); 

Or if generateModel need instance information, you might use new ProductsCollection (or new this ) in your mapToModel to create an instance, then access generateModel on that instance.

 class ProductsCollection { generateModel() { return {name: "testing"}; } static mapToModel(legacy) { const instance = new this(); // or `const instance = new ProductsCollection();` if you want // to use `ProductsCollection` specifically and not be friendly // to subclasses return instance.generateModel(); } } console.log(ProductsCollection.mapToModel({})); 

Answer 2

You cannot access an instance method as it were a static one. This is a classic error. The only way you can use an instance method inside a static one is if you have an instance of the class available (hence the name "instance method").

Usually you are making so mistake in how you are deciding what should be static and what not...without thinking much I can just suggest making the generateModel method static. Another way would be to remove the generateModel method completely and incorporate it's behaviour inside a constructor. It really depends on what your needs are.

Just remember that if you are accessing a non static property inside a method then it should not be static.