How to get data type of variables of a class?

Question

I have class like

 export class Model {
   name: string;
   size: number;
}

if I try to get typeof these variables like below, it returns undefined.

model: Model;
 .
 .
 .
this.model = new Model();
console.log(typeof (this.model.size));

I tried to make class with constructor.

export class Model{        
        constructor(public q: string, public size?: number) {}
}

And I tried to get type of variables.

 this.model = new Model('', null);
 console.log(this.model.size)

It returns object. I tried toType method

toType = function(obj) {
    return ({}).toString.call(obj).match(/\s([a-zA-Z]+)/)[1].toLowerCase()
  }

And

console.log( this.toType (this.model.size));

It returns null.

How can I get variable type correctly without assigning values?

Is this in typescript? Because there's a lot of things in your question that aren't valid JS — coagmano
– coagmano, Commented Apr 16, 2018 at 6:16

Titian Cernicova-Dragomir · Accepted Answer · 2018-04-16 07:15:15Z

You can use the emitDecoratorMetadata and experimentalDecorators option in conjunction with the reflect-metadata package

import 'reflect-metadata'
// Decorator does nothing but metadata is emitted only for decorated symbols by the compiler so we need an empty decorator
function EmitType(target: Object, propertyKey: string | symbol){

}

export class Model {
    @EmitType name: string;
    @EmitType size: number;
    @EmitType child: Model;
    @EmitType data :  { test: string };
}

// Will output function String() { … }
console.log(Reflect.getMetadata("design:type", Model.prototype, "name"));

// Will output function Number() { … }
console.log(Reflect.getMetadata("design:type", Model.prototype, "size"));

// Will output function Model() { … }
console.log(Reflect.getMetadata("design:type", Model.prototype, "child"));

// Will output function Object() { … }
console.log(Reflect.getMetadata("design:type", Model.prototype, "data"));

The value stored as metadata is the type constructor, so for custom classes it will be the constructor for the class. For interfaces and other types that do not have constructors, the type will always be Object

Chirag Ravindra · Accepted Answer · 2018-04-16 06:19:33Z

0

A lot of the code you have shared does not appear to be valid JS. Another pattern I noticed was that you were never actually assigning the values to instance properties in your classes.

Here is a simple example. Hope it helps.

class Test { 
  constructor(a,b){
    // Make sure you assign the values to instance properties
    this.a = a;
    this.b = b;
  }
} 

// Create a new instance passing different typed arguments
var t = new Test('one',1);

// Test the types
console.log(typeof t.a); //string
console.log(typeof t.b); //number

answered Apr 16, 2018 at 6:19

Chirag Ravindra

4,8301 gold badge27 silver badges38 bronze badges

1 Comment

Meg Over a year ago

Yes I know. I want to get type of variables dynamically without assigning values.

Collectives™ on Stack Overflow

How to get data type of variables of a class?

2 Answers 2

Comments

1 Comment

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

Comments

1 Comment

Your Answer

Sign up or log in

Post as a guest

Related