TypeScript编译错误-类型上不存在属性

Question

When I am trying to compile my typescript code the following error appears:

src/Project/ProjectCollection.ts:6:13 - error TS2339: Property 'add' does not exist on type 'BaseCollection'.

I have attached the code below as a demo:

ProjectCollection.ts

export default class ProjectCollection extends BaseCollection {
    addProject(project: Project) {
      super.add(project)
    }
}

BaseCollection.ts

// @ts-ignore
import Collection from 'collectionsjs';

export default abstract class BaseCollection extends Collection {
    public all() {
        return super.all();
    }
}

I am using the following library on github for collections: ( https://github.com/logaretm/collectionsjs )

If I swap the line from:

import Collection from 'collectionsjs'

to:

const Collection = require('collectionsjs')

Then the above error no longer occurs but when running tslint ( https://palantir.github.io/tslint/ ) I get this error:

ERROR: src/BaseCollection.ts:1:20 - require statement not part of an import statement

Just wondering if someone could explain why the first issue with the using import is occurring and how both of these issues can be resolved simultaneously?

Thanks

Answer 1

Just wondering if someone could explain why the first issue with the using import is occurring and how both of these issues can be resolved simultaneously?

The first issue is happening because the collectionsjs module does not have a TypeScript declaration file. As a result, the TypeScript compiler does not have any information about the Collection type. Even though at runtime Collection will have a certain interface, to the compiler the Collection type is somewhat ¹ equivalent to this:

const Collection: any = {};

The way to resolve the issues is to add a type declaration for the collectionsjs module and to create a type that describes what that package exports.

Here is a start of that type declaration file for you:

collectionsjs.d.ts

declare module 'collectionsjs' {
    export default class Collection {
        add(item);
        all();
    }
}

To finish the job, add each of the public members of the Collection class .

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1. It is only "somewhat" equivalent to an any type because though we can extend the Collection we cannot extend the any type.