Customizing the toString Method in a TypeScript Class for JavaScript Objects

I've been struggling with a particular issue for the past few days and I just can't seem to find a solution anywhere. Context: I currently have a TypeScript class that is defined like this:

export class Client extends Person {
    claimNumber: string;
    policyNumber: string;
    address: string;
    insuranceCompany: Organisation = new Organisation();

    toString(): string {
        return this.policyNumber
            .concat(this.claimNumber);
    }
}

This class serves as a model that drives an Angular 5 template. In my component, I use Angular 5's HttpClient to fetch a list of clients from a remote API and generate HTML table rows based on them. The line of code responsible for generating the table rows is:

<tr *ngFor="let client of clients | filter:searchString"> ... </tr>

The searchString mentioned above is a property bound to a search input tag while the filter is a custom filter Pipe defined in the following way:

export class FilterPipe implements PipeTransform {
    transform(items: Client[], term: string) {               
        if (term == undefined || term === '') return items;
        return items.filter(item =>item.toString().toLocaleLowerCase().includes(term.toLocaleLowerCase()));
    }
}

Main Issue: When I inspect item.toString() within the filter pipe, it returns [object Object] instead of a string composed of policyNumber and claimNumber.

Investigation Process: To solve this problem, I conducted a thorough investigation by instantiating the Client class as shown below:

let c = new Client();
c.policyNumber = 'ababababa';
c.claimNumber = 'aaaaaaa';
console.log('client toString() returns => ' + c.toString());

Interestingly, the console.log output was: 'ababababaaaaaaaa'.

Seeking Help: Why does the item.toString() method in the filter pipe yield [object Object], whereas the toString() function on a manually instantiated class gives the correct string?

Answer №1

When retrieving clients from a WebService or similar source, the data comes in the form of plain JSON objects. Even if you classify these objects as type Client, TypeScript will only recognize them based on their properties and not the methods associated with the Client class.

To ensure the objects are treated as actual client instances, it's recommended to instantiate them as such after fetching from the server:

public myServiceMethod() {
    return this.http.get(...).map(plainClients => {
        const realClients: Array<Client> = (plainClients || []).map(plainClient => {
            let realClient = new Client();
            realClient.claimNumber = plainClient.claimNumber;
            realClient.policyNumber = plainClient.policyNumber;
            realClient.address = plainClient.address;
            return realClient;
        });
        return realClients;
    })
}

Alternatively, consider using anemic objects with interfaces defining their types, along with a utility function for extracting client information efficiently:

export interface Person {
    ...
}

export interface Client extends Person {
    claimNumber: string;
    policyNumber: string;
    address: string;
    insuranceCompany: Organisation;
}

// Utility function
public static getClientInfo(client: Client) {
    return client.policyNumber.concat(client.claimNumber);
}

// Usage in your code
return items.filter(item => getClientInfo(item).toLocaleLowerCase().includes(term.toLocaleLowerCase()));

While anemic classes should not be the sole design choice in your application, they can serve well in situations where objects are passed around and likely to be serialized, helping to prevent issues like the one described.

Answer №2

One method to discover the potential issue is by changing the name of your function to something that is not already a predefined method, such as toSearchString. Another helpful step would be to include console logs in your filter function to verify that you are indeed receiving the correct object clients, as it's possible you may be working with a different object.

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