“对象”类型上不存在属性

Question

I am getting various different data structures from external sources. In get() i am checking for undefined and existence of the property, but still getting compiler errors .

Is there a quick fix for that as we are just prototyping at the moment?

class MyCache {
  private map: Map<string, object>;

  constructor() {
    this.map = new Map<string, object>();
  }

  populateMap(uuid: string): boolean {
    // ... getting a JSON object from somewhere
    let externalData: object = {};
    if (uuid == 'abc...') {
      externalData = {"bar": "BAR", "session": uuid}
    } else if( uuid == "def...") {
      externalData = {"foo": "FOO", "session": uuid}
    // ... some more else ifs ...
    } else {
      externalData = {"baz": "BAZ", "session": uuid}
    }
    this.map.set(uuid, externalData);
    console.log(externalData);
    return true;
  }

  get(uuid: string) {
    let response = this.map.get(uuid) || {'session': null};
    if (response.hasOwnProperty('session')) {
      return response.session;
    }
    return '';
  }
}

Error:

Property 'session' does not exist on type 'object'.

Answer 1

AFAIK, type object is too general. I think it is better to use Record<string, unknown> . Please see ts eslint rule :

Unsafe:

//bad
const lowerObj: object = {};

Safe:

// good
const lowerObj: Record<string, unknown> = {};

Using hasOwnProperty is ok, but using Object.prototype.hasOwnProperty.call(foo, "bar") is much better. See eslint rule .

Here you can find recent optimization of V8 engine regarding using hasOwnProperty .

You can find more typings for hasOwnProperty in my article

Hence, this code should be ok:

class MyCache {
    private map: Map<string, Record<string, unknown>>;

    constructor() {
        this.map = new Map<string, Record<string, unknown>>();
    }

    populateMap(uuid: string): boolean {
        // ... getting a JSON Record<string, unknown> from somewhere
        let externalData: Record<string, unknown> = {};
        if (uuid == 'abc...') {
            externalData = { "bar": "BAR", "session": uuid }
        } else if (uuid == "def...") {
            externalData = { "foo": "FOO", "session": uuid }
            // ... some more else ifs ...
        } else {
            externalData = { "baz": "BAZ", "session": uuid }
        }
        this.map.set(uuid, externalData);
        console.log(externalData);
        return true;
    }

    get(uuid: string) {
        let response = this.map.get(uuid) || { 'session': null };
        if (response.hasOwnProperty('session')) {
            return response.session;
        }
        return '';
    }
}

Playground

Answer 2

1. Use Object instead of object link
2. Use 'key' in obj instead of hasOwnProperty link

class MyCache {
    private map: Map<string, Object>;

    constructor() {
        this.map = new Map(); // No need for generic parameters, type is already set in the property declaration
    }

    get(uuid: string) {
        let response = this.map.get(uuid) || { 'session': null };
        if ('session' in response) {
            return response.session;
        }
        return '';
    }
}