After extraction from local storage, the type assertion is failing to work properly

I have a unique situation in my Angular project where I have stored an array of data points in the local storage.

To handle this data, I have created a custom class as follows:

export class Datapoint {
  id: number;
  name: string;
  // ... additional properties.

  constructor(){
  }

  public getDescription() {
    // ... implementation
  }

  // ... more methods
}

Now, when retrieving the array from local storage, I parse it back from a string.

const dpList = JSON.parse(localStorage.getItem('datapoints'));

Since 'dpList' is initially of type 'Object', I perform a type assertion to convert it to my desired type 'Datapoint'.

const datapoints: Datapoint[] = [];

public someFunction(): Datapoint[] {
  // Retrieving the stringified objects from local storage
  const dpList = JSON.parse(localStorage.getItem('datapoints'));

  // Iterating through the array and pushing each element to this.datapoints
  dpList.forEach(dp => {
    const asserted_dp: Datapoint = (dp as Datapoint);
    this.datapoints.push(asserted_dp);
  });
}

However, after the type assertion, 'asserted_dp' seems to be recognized as type Object rather than Datapoint. This prevents me from accessing the designated functions of type Datapoint since the proto property does not contain such information.

If anyone has a solution on how to properly execute the type assertion, I would greatly appreciate your advice!

Thank you in advance!

Answer №1

Comprehending the Problem

It's important to note that local storage is limited to storing strings only. Even if you have converted your Datapoint objects into strings for storage, using JSON.parse to retrieve the data will not give you back a Datapoint instance, but rather a regular JavaScript Object. This means that your object loses its Datapoint methods such as getDescription, and you cannot simply cast the type.

Potential Resolution

To address this issue, I suggest creating a loader function that can reconstruct instances of your Datapoint from the serialized string. While I'm not familiar with how you usually create your data points, here's a basic example:

function loadDatapoints(): Datapoint[] {
    var dp_objects: any = JSON.parse(localStorage.getItem('datapoints'));
    var points: Datapoint[] = [];

    for (var i: number = 0; i < dp_objects.length; i++) {
        var point: Datapoint = new Datapoint();
        // Implement your typical Datapoint construction logic here...
        points.push(point);
}

return points;
}

By following this approach, you'll be able to work with actual Datapoint instances, allowing type assertion to function correctly.

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