Retrieve all the characteristics accessible of a particular course

Question

Retrieve all the characteristics accessible of a particular course

I am facing a situation where I have the following class structure:

class A {
  id: number
  propertyA: string

  constructor(id: number) {
    this.id = id
  }
}

let a = new A(3)

console.log(SomeFunction(a))
// expected output = ['id', 'propertyA']

Essentially, when an instance of class A is created, the value for propertyA is not set. However, when SomeFunction(a) is called, it should return all properties that class A can have, including propertyA even if it has not been explicitly set.

Object.getPropertyNames(a) only returns ['id']

What would be the code implementation for SomeFunction?

typescript typescript-generics

Answer 1

Answer №1

To ensure a property exists on an object, you must assign a value to it. At runtime, there is no metadata indicating that a property has been declared but never assigned.

If you want every instance of A to have the property propertyA, you should set a default value for it, such as in an initializer on the declaration.

For example, without strict null checks, you could do:

class A {
  id: number
  propertyA: string = undefined

  constructor(id: number) {
    this.id = id
  }
}

This way, new A(42).propertyA will return undefined, but the property will exist and can be accessed by SomeFunction.

If you are using strict null checks, initialize propertyA to a string.

Answer 2

To ensure a property exists on an object, you must assign a value to it. At runtime, there is no metadata indicating that a property has been declared but never assigned.

If you want every instance of A to have the property propertyA, you should set a default value for it, such as in an initializer on the declaration.

For example, without strict null checks, you could do:

class A {
  id: number
  propertyA: string = undefined

  constructor(id: number) {
    this.id = id
  }
}

This way, new A(42).propertyA will return undefined, but the property will exist and can be accessed by SomeFunction.

If you are using strict null checks, initialize propertyA to a string.

Answer 3

Answer №2

In order for a property to exist within a class instance, its value must be set.

When enumerating the properties id and propertyA, they will only show up if they have been assigned a value:

function listProperties(obj) {
  return Object.keys(obj);
}

class B {
  constructor(id, propA) {
    this.id = id;
    if (propA)
      this.propertyA = 0;
  }
}

const x = new B(5);
const y = new B(5, 2);

console.log(listProperties(x));
console.log(listProperties(y));

Answer 4

In order for a property to exist within a class instance, its value must be set.

When enumerating the properties id and propertyA, they will only show up if they have been assigned a value:

function listProperties(obj) {
  return Object.keys(obj);
}

class B {
  constructor(id, propA) {
    this.id = id;
    if (propA)
      this.propertyA = 0;
  }
}

const x = new B(5);
const y = new B(5, 2);

console.log(listProperties(x));
console.log(listProperties(y));

Retrieve all the characteristics accessible of a particular course

Answer №1

Answer №2

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