TypeScript: Deserializing multiple types from a single object

Question

I have a JSON array of serialized objects that each have a type field. I'm trying to deserialize them but can't get TS to play along:

ts playground link

type serializedA = { type: 'A', aProp: string };
class A {
    public type = 'A';
    constructor(data: serializedA) {}
}

type serializedB = { type: 'B', bProp: string };
class B {
    public type = 'B'
    constructor(data: serializedB) {}
}

const classMap = { A, B };

function deserialize(serializedObject: serializedA | serializedB) {
    const Klass = classMap[serializedObject.type];
    new Klass(serializedObject);
}

The issue is that last line. TS doesn't know that Klass and serializedObject are now either both A or both B, and I'm having trouble figuring out how to tell it that.

The issue is that the construct signature of Klass is new (serializedA & serlializedB) => A | B which reduces to new (never) => A | B since the type {type: 'A' & 'B', aProp: string, bProp: string} must be empty. This is definitely annoying since you know better, but you have to either use a type assertion or write a series of tests that narrow the type to its individual possibilities. — Aluan Haddad
– Aluan Haddad, Commented Sep 15, 2020 at 22:59

Pritam Kadam · Accepted Answer · 2020-09-16 05:08:48Z

1

For production grade decoding libraries, take a look at following:

we are using io-ts extensively. You can refer it here

answered Sep 16, 2020 at 5:08

Pritam Kadam

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1 Comment

Souperman Over a year ago

I would second io-ts as an excellent option for this. We also use it exclusively

Cenk Çetinkaya · Accepted Answer · 2020-09-15 20:50:15Z

0

type serializedA = { type: 'A', aProp: string };
class A {
    public type='A';
    constructor(data: serializedA) {}
}

type serializedB = { type: 'B', bProp: string };
class B {
    public type ='B'
    constructor(data: serializedB) {}
}

const classMap = { A, B };

function deserialize(serializedObject: serializedA | serializedB) {
    if(serializedObject.type === "A"){
        let Klass = classMap[serializedObject.type]
        return new Klass(serializedObject);
    } else {
        let Klass = classMap[serializedObject.type]
        return new Klass(serializedObject);
    }
}

answered Sep 15, 2020 at 20:50

Cenk Çetinkaya

1396 bronze badges

1 Comment

user578895 Over a year ago

Yeah, I'm trying to avoid that ;) There are about 50 different classes and I don't like writing super verbose code just to make TS happy

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