Exploring the automatic type inference functionality of Typescript for generic classes

Although it may seem simple, I am struggling to pinpoint the cause of this error. I have been searching for a solution for quite some time, but I have yet to find one.

class MyClass<T > {
  property: T = 5 // Error Here: Type '5' is not assignable to type T
}

const tester = new MyClass<number>() // OK

const numberValue: number = tester.property // OK
const stringValue: string = tester.property // Error as expected

Initially, I assumed that T would be inferred as a number based on the 'property' value. I vaguely remember this working in the past, but I am not entirely certain.

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Update

Even after implementing these definitions, I am still encountering the same errors.

class MyClass<T extends number> {
  property: T = 5 // Error Here: Type '5' is not assignable to type T
}

class MyClass<T extends object> {
  property: T = {} // Error Here: Type '{}' is not assignable to type T
}

Answer №1

It is truly remarkable that this approach was successful. Writing the definition of a generic class or method requires consideration of every possible value of T that the class or method may encounter. Assigning property: T = 5 would not be appropriate if an instance of MyClass<string>() is created, resulting in T being a string. Inference takes place only when a generic class or method is actually utilized.

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