form_field_values.ts

import { Preconditions, UnreachableError } from 'base/preconditions';

export module FormFieldValue {
  export const enum Type {
    Text,
    Select,
    OnOff,
    Numeric,
  }

  function getNameForType(type: Type): string {
    switch (type) {
      case Type.Text:
        return 'string';
      case Type.Select:
        return 'string or undefined';
      case Type.OnOff:
        return 'boolean';
      case Type.Numeric:
        return 'number';
      default:
        throw new UnreachableError(type);
    }
  }

  export type Text = { type: Type.Text; value: string };
  export type Numeric = { type: Type.Numeric; value: number };
  export type OnOff = { type: Type.OnOff; value: boolean };
  export type Select = { type: Type.Select; value: string | undefined };

  export type T = Text | Numeric | OnOff | Select;

  export type Primitive = T['value'];

  export function text(value: string): Text {
    return { type: Type.Text, value };
  }

  export function onOff(value: boolean): OnOff {
    return { type: Type.OnOff, value };
  }

  export function numeric(value: number): Numeric {
    return { type: Type.Numeric, value };
  }

  export function select(value: string | undefined): Select {
    return { type: Type.Select, value };
  }

  export type PrimitiveForType<DesiredType extends Type> = Extract<T, { type: DesiredType }>['value'];

  export function getAsType<DesiredType extends Type>(
      formFieldValue: T,
      type: DesiredType,
  ): PrimitiveForType<DesiredType> {
    Preconditions.checkArgument(
        formFieldValue.type === type,
        `Expected field value of type ${
            getNameForType(type)
        }, got field value of type ${
            getNameForType(formFieldValue.type)
        }. You probably have two form fields with the same label.`,
    );
    return formFieldValue.value;
  }

  export function getDefaultValue(type: Type) {
    switch (type) {
      case Type.Numeric:
        return 0;
      case Type.OnOff:
        return false;
      case Type.Select:
        return undefined;
      case Type.Text:
        return '';
      default:
        throw new UnreachableError(type);
    }
  }
}