Is it possible to initialize an object inside another object initialization without constructors?

Question

What I need is fairly simple, I just want to initialize an object that is a child of another object which I am initializing. In C# it would look something like this:

var car = new Car { 
serialNumber = 25,
engine = new Engine {
    horsePower = 500
    }
}

In this example code I am initializing the Engine type inside the initialization of the Car type. While trying to do this with TypeScript no matter what I try I get syntax errors.

Here is my TypeScript code:

var car: Car = new Car({
serialNumber: 25,
    engine: new Engine({ 
        horsePower: 500
    })
});

This block of code just does not work, and all the classes used in the code here are obviously imported.

Could it really be a basic feature like this isn't supported in TypeScript?

EDIT: There is no constructor in these classes. Here's how they look:

export class Car { 
    serialNumber: number;
    engine: Engine;
}

If that piece of code is giving you errors, I would just suggest to initialize "Engine" before the initialization of Car and then use it on the Car class initialization ;) — gascon95
– gascon95, Commented Mar 1, 2017 at 1:03
@gascon95 This code is only for reference, In my code it is far more complicated, I have multiple objects to initialize and I find it time consuming and ineffective to initialize each object beforehand and create a variable. The solution is obviously easy, but the question is if that really is the only way? — Bodokh
– Bodokh, Commented Mar 1, 2017 at 1:05

Aaron Beall · Accepted Answer · 2017-03-02 14:04:53Z

8

You can certainly do this, you just have to pass the correct constructor arguments.

The code you've written passes a single constructor argument object with keys. So your classes should look something like this:

class Car {
    constructor(args: { serialNumber: number; engine: Engine; }) { }
}

class Engine {
    constructor(args: { horsePower: number; }) { }
}

If, however, your classes use multiple named constructor arguments, like this:

class Car {
    constructor(serialNumber: number, engine: Engine) { }
}

class Engine {
    constructor(horsePower: number) { }
}

Then you simply have to pass your constructor arguments correctly:

const car: Car = new Car(25, new Engine(500));

EDIT

that would require me to create a constructor which is kind of annoying

I'm a bit confused... your original code uses classes, so you should already have a constructor unless you are adding each property using an assignment. Please post a complete example.

If you're just trying to create a nested object literal structure, you just need an interface, then create the object using normal JS object literal notation:

interface Car {
  serialNumber: number;
  engine: Engine;
}

interface Engine {
  horsePower: number;
}

const car: Car = {
  serialNumber: 25,
  engine: { 
    horsePower: 500
  }
};

If you're trying to assign an object literal to a class type, then you're doing it wrong. :)

EDIT 2

So, you have a class with properties and no constructor. This means you can't instantiate it with properties, either nested or one at a time. You could write a generic helper function to instantiate a class and assign properties:

function create<T>(constructor: new() => T, props: Partial<T>): T {
    let instance = new constructor();
    for (let key in props) {
        instance[key] = props[key];
    }
    return instance;
}

And then create a nested class object like this:

const car = create(Car, {
    serialNumber: 123,
    engine: create(Engine, {
        horsePower: 456
    })
});

But if your classes really just have properties, I think you'd be better off just using interfaces and object literals (as in my previous example).

edited Mar 2, 2017 at 14:04

answered Mar 1, 2017 at 4:14

Aaron Beall

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7 Comments

Bodokh Over a year ago

I added an example code of the class, And i'll try using interface and see what happens.

Aaron Beall Over a year ago

Ok. I see your class has properties but no constructor. This means you can't instantiate it with properties, either nested or one at a time. Is your question really "how do I initialize a class instance with properties without using a constructor?"

Bodokh Over a year ago

Yeah I guess so, sorry for misleading. Also I found something else that is dumb, if i have a constructor without parameters and initialize the object the constructor code won't fire. The more i use typescript the more I hate it :/

Aaron Beall Over a year ago

if i have a constructor without parameters and initialize the object the constructor code won't fire -- that's definitely not true, something else must be wrong with your code.

Bodokh Over a year ago

It sounds retarded but I'm telling you, that's how it is. After adding a useless parameter just to verify the constructor code works.

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Amir H. Bagheri · Accepted Answer · 2017-03-01 01:10:53Z

0

Your answer is definitely you can. But you syntax has a probelm.

var car = new Car(
       serialNumber: 25,
      engine: new Engine{ 
      horsePower: 500
     }
 );

and the Car class should be like this:

public class Car(int serialNumber, Engine engine) {

 .....
}

edited Mar 1, 2017 at 1:10

answered Mar 1, 2017 at 1:06

Amir H. Bagheri

1,4061 gold badge11 silver badges18 bronze badges

6 Comments

Bodokh Over a year ago

I'm afraid I'm still getting the same error: Supplied parameters do not match any signature of call target.

Amir H. Bagheri Over a year ago

As I said, you Car class structure should look like above. What is the Car class structure?

Amir H. Bagheri Over a year ago

I modified the post, I made a mistake in writing that.

Bodokh Over a year ago

It makes no difference and i don't see how it would've made a difference.

Amir H. Bagheri Over a year ago

As I said, there is an issue with class constructor. The class could simply have no constructors. Check that. You may copy it here.

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