Description

Simple, lightweight type casting of JSON objects to classes. Use it on the JSON response from your server to so you can use proper model classes on the frontend in one function call.

Your server has model classes. Your frontend has model classes. But, there isn't a great way to get instances of your Javascript model classes from the JSON objects in your responses. This package gives you an easy way to do so, without integrating with a complex Javascript framework, tying you to a specific server framework, or pulling in many dependencies (this package has none).

Example

Given a JSON structure that looks like this:

var homer = {
  name: "Homer Simpson",
  __type: "Person",
  foods: [
    { name: "Jelly", category: "donut", favorite: true, __type: "Food" }
  ]
}

and some model classes that look like this:

@Register('Person')
class Person {
  getFavoriteFood() { return this.foods.find(f => f.favorite); }
}

@Register('Food')
class Food {
  getFullName() { return `${this.name} ${this.category}`; }
}

you can easily cast your JSON objects to instances of their respective classes:

autoTypeCast(homer);
homer.getFavoriteFood().getFullName(); // "Jelly donut"

Does this function mutate the original objects? Yes, and it uses setPrototypeOf to do so. Mozilla claims this is much slower than Object.assign, and also a dangerous operation. My benchmarks show the opposite, but I am actively looking for counterexamples.

Doesn't this mean I need to add an attribute to my JSON objects being sent from the server? Yes. This is simple enough for my use cases. If this does not fit for you, you should find a more complex framework. For example, class-transformer is non-intrusive through use of type decorators.

TypeScript Support

This library includes full TypeScript support while maintaining its simple, prototype-based transformation approach. A few important notes about types:

1. The autoTypeCast function returns unknown by design:

   function autoTypeCast(obj: unknown, options?: AutoTypeCastOptions): unknown;

   This is intentional because the function transforms objects by changing their prototypes at runtime. The return type being unknown correctly represents that the object's type changes during execution rather than at compile time.

2. Typical Usage Pattern: The library is designed to be used at a fundamental level in your application, such as in an axios interceptor:

   axios.interceptors.response.use((response) => {
     autoTypeCast(response.data);
     return response;
   });

   In this pattern, the unknown return type doesn't matter because:

   • The underlying object is transformed in place
   • Your type system already knows the expected type from your API definitions
   • The object will have the correct prototype and all instance methods at runtime

3. The decorators are fully typed:

   @Register("Person")
   class Person {
     @Transform((value: string) => value.toUpperCase())
     name: string;
   }

Usage

Installation

First, add the package to your project:

npm install auto-type-cast --save

Next, register your model classes using the @Register decorator:

import { Register } from 'auto-type-cast';

@Register('Person')
class Person {
  // ... your class implementation
}

The @Register decorator takes a string parameter that specifies the type name to use for this class. This is particularly useful when your class names might be minified or obfuscated in production, as it allows you to maintain a consistent type name regardless of code transformation.

Make sure your server sends a __type attribute matching the registered name, or augment the Javascript once you have it on the client. You're on your own with this, but here's an example from Ruby using ActiveModelSerializers:

class ApplicationSerializer < ActiveModel::Serializer
  attribute(:__type) { model.class.name }
  #assumes model class on server has same name as registered on frontend
end

class PersonSerializer < ApplicationSerializer; end

Configure autoTypeCast. See Configuration below.

Finally, call autoTypeCast on the object. Arrays and deeply-nested structures will be scanned and cast as well, if found.

import autoTypeCast from 'auto-type-cast';

var response = fetch(...);
autoTypeCast(response.data);

Alternative Registration Methods

While using the @Register decorator is the recommended approach, you can also register classes manually:

import { registerClass } from 'auto-type-cast';

class Person {
  // ... your class implementation
}

registerClass(Person);  // Will use the class name as the type

Property Transformations

You can use the @Transform decorator to automatically transform property values during type casting. This is particularly useful for converting date strings to Date objects, formatting strings, or transforming nested data structures:

import { Register, Transform } from 'auto-type-cast';

@Register('Person')
class Person {
  @Transform(date => new Date(date))
  birthDate;

  @Transform(str => str.toUpperCase())
  name;
}

const data = {
  __type: 'Person',
  birthDate: '1990-01-01',
  name: 'Homer Simpson',
};

autoTypeCast(data);
// data.birthDate is now a Date object (1990-01-01)
// data.name is now 'HOMER SIMPSON'

The Transform decorator takes a function that receives the property's value and returns the transformed value. The transformation is applied during the type casting process, after the object's prototype is set but before any afterTypeCast hooks are called.

Transform Error Handling

Transform functions are executed in a safe manner - if a transform throws an error, the original value is preserved and the type casting process continues. This ensures that your application won't break even if a transform fails:

@Register("User")
class User {
  @Transform((value) => new Date(value)) // If this fails, original string is kept
  createdAt;

  @Transform((str) => str.toUpperCase()) // This will still run even if createdAt transform failed
  name;
}

You can configure how transform errors are handled either globally or per-call:

// Global error handling configuration
import config from "auto-type-cast/config";

config.onTransformError = (error, propertyKey, value, transformFn, type) => {
  ErrorMonitoring.capture(error, {
    context: {
      type, // The class type (e.g., 'User')
      property: propertyKey, // The property that failed (e.g., 'createdAt')
      value, // The original value that caused the error
    },
  });
};

// Per-call error handling
autoTypeCast(data, {
  onTransformError: (error, propertyKey, value, transformFn, type) => {
    console.warn(`Transform failed for ${type}.${propertyKey}:`, error);
  },
});

The error handler receives:

• error: The error that was thrown
• propertyKey: The name of the property being transformed
• value: The original value that caused the error
• transformFn: The transform function that failed
• type: The class type name

Customization

You can control the name of the attribute that specifies the class name:

var myObject = { "name": "Homer", myType: 'Person' };
autoTypeCast(myObject, { typeKey: 'myType' });
// myType was used to convert the object to instance of Person

If you need more granular or dynamic control of the class registry, import it directly:

import { classRegistry } from 'auto-type-cast';
delete classRegistry['Person'];
autoTypeCast({ __type: 'Person'}); // no conversion

Configuration

Parameter	Description	Default
config.typeKey	Determines the attribute name that specifies the object's type	__type
config.getObjectType	Returns the object's type. Used to look up the correct class in the registry. Uses config.typeKey by default, but if you need more control, you can override this	(object, options) => object[options.typeKey || config.typeKey]
config.getClassType	Returns the class's "type" name. Used to map objects to this class. The object type and the class type must match in order for autoTypeCast to work.	(klass) => klass.registeredName || klass.name
config.beforeTypeCast	Called with the object as a parmeter immediately before type casting it. You can use this if you need to transform it (e.g. manipulating __type) before it is ready for type cast.	No-op (object) => {}
config.afterTypeCast	Called with the object as a parmeter immediately after type casting it. You can use this if you need to transform it after it has taken on its new class (e.g. calling a function from the new class like object.mySpecialInit())	No-op (object) => {}
config.onTransformError	Called when a transform function throws an error. Receives the error, property key, original value, transform function, and class type.	Logs warning and preserves original value

Development

Testing

• npm run watch
• Edit base code and observe test results

Building

• npm run prod Runs tests, lints, and builds the module
• npm version patch (or minor/major etc.)
• npm publish