Syntax for declaring defaults

Question

I have been experimenting with OO in typescript lately, and have been trying to declare some generic classes.

I am trying to extend a class, and declare defaulted values inside the class itself, but I am running into a syntax error.

Why would my property be read only in this situation?

    export interface Heading {
      size: number;
      color: number;
    }

    export class text {
      size: number;
      color: number;
    }

    export class Document extends text {
      H1: Heading = {
        size = 1 // syntax error - Cannot assign to 'Number' because it is a constant or a read-only property.
      };
    }

Reactgular · Accepted Answer · 2017-07-14 13:41:36Z

2

You have two issues.

First, the color attribute has to be marked as optional if you are going to exclude it.

export interface Heading {
    size: number;
    color?: number;
}

Second, you have to use Object notation : and not equals.

export class Document extends text {
    public H1: Heading = {
        size:  1
    };
}

answered Jul 14, 2017 at 13:41

Reactgular

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Comments

lilezek · Accepted Answer · 2017-07-14 13:44:20Z

If you want your Document to have default values of size and color, you should write the values as they appear in the extended class text:

export class Document extends text {
  size = 1;
  color = 1;
}

By the way, you code had an issue:

TypeScript compiler really says H1 lacks of color property. So, you either add it to your class:

export interface Heading {
  size: number;
  color: number;
}

export class text {
  size: number;
  color: number;
}

export class Document extends text {
  H1: Heading = {
    size: 1,
    color: 1,
  };
}

or place an optional ? operator:

export interface Heading {
  size: number;
  color?: number;
}

export class text {
  size: number;
  color: number;
}

export class Document extends text {
  H1: Heading = {
    size: 1
  };
}

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