TypeScript Type Inheritance : How to assign variable with parent type to the variable with child type

user_3007690 0
Viewed 1211

I have created two types 

export class GenericType {
  public id: string;
  public property: string;
  public name: string;
}
export class SpecificType extends GenericType {
  public value: string;
}

Now I have two variable A: GenericType and B: SpecificType, now I want to assign all the values of A to B, and additionally, I want to assign value to B. How should I achieve this, I don't want to loop through A's all property and manually assign it to B's all property.

let varA: GenericType = {
  id: '123',
  property: 'exmple',
  name: 'example variable',
};

let varB: SpecificType = new SpecificType();
varB = varA;  <----- not allowing to assign.
varB.value = 'new value';
typescript
2 Answers

You cannot do this:

let varA: GenericType = {
  id: '123',
  property: 'exmple',
  name: 'example variable',
};

let varB: SpecificType = new SpecificType();
varB = varA;  <----- not allowing to assign.
varB.value = 'new value';

But you can do this:

let varA: GenericType = {
  id: '123',
  property: 'exmple',
  name: 'example variable',
};

let varB: SpecificType = new SpecificType();
// Difference here:
varB = { ...varA, value: 'new value' }

You cannot assign varA to varB because varA does not have value, but you can assign everything altogether using the spread operator.

user_1927290

You're not allowed to assign it because the property value does not exist in the GenericType class, to circumvent this you can do a type assertion to SpecificType:

let varB: SpecificType = new SpecificType();
varB = varA as SpecificType; // Allowed to assign now.
varB.value = 'new value';
Admin1
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