How can we reduce an object to conform to an interface in TypeScript?

Object Reduction to Interface Conformity

In TypeScript, an interface defines a contract for an object's properties, while a class implements that interface. However, it's common to encounter objects with additional properties beyond those defined in an interface, which can pose challenges when using tools like JSON.stringify() that convert objects to JSON.

To address this, it's necessary to reduce an object such that it only contains properties from the interface it implements.

Problem:

When attempting to convert an object with additional properties to JSON using JSON.stringify(), the extra properties are included in the output, potentially invalidating the JSON.

Solution 1:

Due to the fact that TypeScript interfaces have no runtime representation and thus cannot be used to enforce property restrictions, a workaround is needed. By defining the interface as a class, we can use a library like Lodash to pick only the properties defined in the interface from the target object.

For example:

<code class="typescript">import _ from 'lodash';

const reducedObj = new MyInterface();
_.assign(reducedObj, _.pick(originalObj, _.keys(reducedObj)));</code>

This method ensures that the reduced object contains only the properties defined in the interface.

Solution 2:

As another approach, one can manually loop through the target object's properties, checking if each property exists in the interface definition. If it does, the property is added to the reduced object. This method requires careful attention to avoid adding unintended properties.

While there is no direct way to enforce property conformity with TypeScript interfaces in JavaScript, these workarounds provide practical solutions to reduce objects to conform to interface definitions.

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