I am getting an error that says, "Cannot add property count; the object is not extensible" while running LWC [duplicate]. Why is that?

It seems that the response from Apex cannot be extended with additional parameters with Spring 20. Which means that the object is not extensible https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Object/preventExtensions For objects, you can use either result = object.assign({}, result)or item = object.assign({}, item)as a workaround for every item in result for arrays in a loop. You can also spread syntax data = { ...result };. There is another dirty hack, JSON.parse(JSON.stringify(result));.