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Tolerim
a month ago

How can I utilize Jest to test for strict equality in an object instantiated through a class?

In this code snippet, there is a class called DummyClass which creates an object using the new keyword. While testing this class using Jest, the coder is attempting to compare an instance of DummyClass to an object. However, since the object is not an instance of DummyClass, the test fails. The coder is seeking a solution to modify the object to an instance of DummyClass without making individual assertions due to the size of the data.
Answers(2)
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Vogi
a month ago
Verified Answer
You can use the Object.assign() method to create a new object with the same properties as the original object. Then you can use the Object.setPrototypeOf() method to set the prototype of the new object to the prototype of the class you want to compare it to. Here's an example: 
const dummyClassInstance = new DummyClass();
const object = { ...dummyClassInstance };
Object.setPrototypeOf(object, DummyClass.prototype);
expect(object).toBeInstanceOf(DummyClass);
This creates a new object with the same properties as dummyClassInstance, sets its prototype to DummyClass.prototype, and then checks if it is an instance of DummyClass using Jest's toBeInstanceOf() method. I hope this helps! Let me know if you have any other questions.

Sources

CodeTip - Javascript: Compare class instances - DEV Communityhow to test if an object is an instance of class in jestComparing two instances of a class in Java - Stack OverflowHow do I compare two instances of a class? – ITExpertly.comCompare objects containing anonymous functions on jest testcase
avatar
Tolerim
a month ago
You can create a new instance of DummyClass with the data you want to compare and then use that instance in your Jest test. Here's how you can modify your test: 
import DummyClass from '../dummy-class.js';

it('should have the same data', () => {
  const data = new DummyClass({
    name: 'John',
    age: 23,
    datetime: 'someDate'
  });

  const instance = new DummyClass(data);

  expect(data).toStrictEqual(instance);
});
In this example, we first create a new instance of DummyClass with the data we want to compare. Then, we create another instance with the first instance as its input, effectively creating a deep copy. Finally, we use the first instance and the second instance in the toStrictEqual assertion. This should now pass the test as both objects contain the same data and are instances of the same class.
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