typescript initialize object with partial object literal

Question

I want to initialize an instance of a class with an object literal not containing all the elements in the class but whose elements are in the class.

class ATest{
    aStr:string;
    aNum:number;

    result(){return this.aStr + this.aNum;}
}
let test=new ATest;

test={aStr:"hello",aNum:12};   // error, result() is missing in the object literal

What is the way to do that assignment exploiting the most of the validations of ts?

test=Object.assign(test,{aStr:"hello",aNom:12});

would work but then you miss the validation of the input fields - see aNom that's wrong but gets in

Titian Cernicova-Dragomir · Accepted Answer · 2018-02-05 13:30:19Z

16

You have an object literal, an instance of a class must be created using new ClassName()

The simplest solution is to add a constructor accepting a Partial<T>

class ATest{
    aStr:string;
    aNum:number;
    constructor(obj: Partial<ATest>) {
        Object.assign(this, obj);
    }
    result(){return this.aStr + this.aNum;}
 }

 let test=new ATest({aStr:"hello",aNum:12});

Inside this new constructor you can do validations of the fields as required.

edited Feb 5, 2018 at 13:30

answered Feb 5, 2018 at 12:12

Titian Cernicova-Dragomir

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6 Comments

tru7 Over a year ago

Yes, but the validations would be at runtime. I was looking for a solution detecting at compile time if that's possible...

Titian Cernicova-Dragomir Over a year ago

@Joan, The Partial<T> is validated at compile time. It forces you to specify members of T with the same type. I thought you wanted some extra validations, such as weather aNum >0 or the like. Do you want all the members to be mandatory ?

tru7 Over a year ago

Oh, I was totally unaware of the Partial<> thing!

tru7 Over a year ago

Thanks! I see that this would not work on a non-literal object: let prms={aStr:"hello",aXXXXNum:12}; let test=new ATest(prms); lets it pass without warning... or I'm missing something?

Titian Cernicova-Dragomir Over a year ago

@Joan no unfortunately that is true, this is not due to partial though, excess properties on object literals are only checked on assignment of a literal to a variable. So let prms: Partial<ATest>={aStr:"hello",aXXXXNum:12}; let test=new ATest(prms); is invalid, but your example is valid. Type checks still occur so you can't pass let prms={aStr:"hello",aNum:'12'}; let test=new ATest(prms);

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