modr.js

export type Path = string[];
export type Modifier = (data: any, path: Path) => any;
export type Schema = { [key: string]: Modifier };

export const defaultValue = (value: any): Modifier =>
  (data) =>
    data === undefined ? value : data;

export const overrideValue = (value: any): Modifier => () => value;

export const modifyType = (type: any, modifier: Modifier): Modifier =>
  (data, path) => (
    data !== undefined && data !== null && data.constructor === type ?
    modifier(data, path) :
    data
  );

export const modifySchema = (schema: Schema) =>
  modifyType(Object, (data, path) =>
    Object.keys(schema).reduce((newData, key) => ({
      ...newData,
      [key]: schema[key](data[key], [...path, key]),
    }), {})
  );

export const modifyCollection = (modifier: (index: number) => Modifier) =>
  modifyType(Array, (data, path) =>
    data.map((elem: any, index: number) =>
      modifier(index)(elem, [...path, index.toString()])
    )
  );

export const composeModifiers = (modifiers: Modifier[]): Modifier =>
  (data, path) =>
    modifiers.reduce((result: any, modifier: Modifier) =>
      modifier(result, path)
    , data);

modifySchema({
  foo: defaultValue('hello')
})({}, [])
// Object {foo: "hello"}

modifySchema({
  foo: defaultValue('hello')
})({foo: 'bar'}, [])
// Object {foo: "bar"}