How to initialize a variable in a class to its generic value using Typescript

Question

I am building a component to hold information about fields for use in a form. I need to hold various data objects to create generic routines for working with the data.

export class DataField<T> {
    /**
     * Data field name IE: database column name, JSON field, etc.
     */
    private Name_: string;
    private Size_: number;
    /**
     * The data type for this field. IE: String, Boolean, etc. Note that this is set internally, and is not set by a passed reference
     */
    private Type_: T;
    // private Type_: { new(): T ;}   <- this has not worked either

    /**
     * Class constructor
     * @param FieldName string Name of the Field
     * @param FieldSize number Field Size
     */
    constructor(FieldName:string, FieldSize:number) {
        this.Name_ = FieldName;
        this.Size_ = FieldSize;
    }

    /**
     * Get the data type for the value
     * @returns string The TypeOf the value
     */
    get Type(): string {
        return (typeof this.Type_).toString();
    }
}

The problem that I have is that the field Type_ is not initialized to anything, so it is always set as undefined. When I create the DataField with the generic, it might look like:

new DataField<string>('FullName', 32);

The generic type T is now string, but I want to be able to get the Type_ variable set so the call to get Type() will return a string.

Type_ is never assigned a value... so how can it be anything but undefined? What do you expect to be the content of Type_? — Sébastien
– Sébastien, Commented Jan 22, 2018 at 23:24
If you are asking about how to get a default value, when you have only T as generic type parameter - the answer is you can't, type parameters are erased from generated javascript. See also stackoverflow.com/questions/42823198/… and stackoverflow.com/questions/24469856/… — artem
– artem, Commented Jan 23, 2018 at 0:11
@Sébastien I return a string value, such as 'boolean', 'number', 'string', etc. from Type_. I want this to tell me the type of data. You are correct in that it is not initialized to anything, which is what I am trying to do, so that I can get the type from it. — Steven Scott
– Steven Scott, Commented Jan 23, 2018 at 16:27
"I want this to tell me the type of data" the type of data of which data? If you never give a value to the Type_ property, then it has no other type than undefined. I guess I still don't understand what you are trying to do... — Sébastien
– Sébastien, Commented Jan 23, 2018 at 16:48

Greg Rozmarynowycz · Accepted Answer · 2018-01-23 01:47:45Z

4

You'll need to pass the type as a parameter to your constructor; in TypeScript there's no need explicitly set it as a generic:

class DataField<T> {

    constructor( 
        private type: {new (...args): T}, 
        private fieldName: string, 
        private fieldSize: number) {}

    public getType(): string {
        return this.type.name;
    }
}

const field = new DataField(String, 'name', 256);
console.log(field.getType()); // outputs "String"

Remember classes in JS are no more than functions, the class declaration is syntactic sugar. "Types" in JavaScript are no more than the constructor (as in the function object itself) that when called with new will created an object with the desired prototype.

Note you really don't need a generic as your current class is built (I just left it for sake of example); it would function just fine without it. You could replace {new (...args): T} with {new (...args): any}. You would need a generic if you wanted a data container class that returned actual values with the correct type:

class DataContainer<T> {
   // other class stuff

   public getValue(): T {
      // do stuff to return value
   }
}

answered Jan 23, 2018 at 1:47

Greg Rozmarynowycz

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4 Comments

Steven Scott Over a year ago

I guess there is no way to do it with the Generic, and not have to pass the type again to the constructor on the new DataField(TYPE, 'name', 256) where TYPE is boolean, number, etc.?

Steven Scott Over a year ago

I guess the issue is that if I pass the type as you suggest String (which Visual Studio Code warns is a type being used as a value) then I could just as easily pass the text representation 'string' and simply the constructor even further. I was really hoping to be able to do the same thing, but use the generic type, without the extra constructor variable. I guess this cannot be done at the current time with Typescript. I know that Javascript will not support this, but it was mostly for the type checking in Typescript and then library handling afterwards.

Greg Rozmarynowycz Over a year ago

If TypeScript did support that it would just be syntactic sugar for a ctor variable. Note there is a big difference between string and String: string is a primitive JavaScript value type (of number, boolean, object, and undefined, among others), and String is a native function (aka class). As an symbol, string only exists in TypeScript. Note you should not use native classes (String, Number, etc.) to create new primitive values: 35 !== new Number('35')

Steven Scott Over a year ago

I am not using the String, but string, but yes, the syntactic sugar would be nice.

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