Lenses in JS

README.md

Lenses in JS

From "Mastering JavaScript Functional Programming"

lenses_built_from_objects.js

/**
 * These are not lenses
 */
const getField = attr => obj => obj[attr];
const setField = attr => val => obj => ({ ...obj, [attr]: val });

// ----------------------------

/**
 * A lens is an object with a `getter` and a `setter` method.
 * - Getter takes an object, and returns the value of one of the object's properties
 * - Setter takes a value and an object, and sets that value on one the object's properties
 */

const createLensFromGetterAndSetter = (getter, setter) => ({ getter, setter });

const createLensFromPropertyName = attr => createLensFromGetterAndSetter(getField(attr), setField(attr));

/**
 * 1st of the 3 basic functions to use lenses with
 */
const view = lens => obj => lens.getter(obj);

/**
 * 2nd of the 3 basic functions to use lenses with
 */
const set = lens => value => obj => lens.setter(value)(obj);

/**
 * 3rd of the 3 basic functions to use lenses with
 */
 const over = lens => mappingFn => obj => {
  const newValue = mappingFn(view(lens)(obj))

  return lens.setter(newValue)(obj);
 };

// ----------------------------

const myObj = {
  name: 'JF',
  bird: 'wood pigeon'
};

const nameLens = createLensFromPropertyName('name');
const viewName = view(nameLens);
const setName = set(nameLens);
const overName = over(nameLens);

console.log(
  // viewName(myObj) // JF
  // view(nameLens)(myObj) // JF
  // setName('Hiro')(myObj) // { name: 'Hiro', bird: 'wood pigeon' }
  // set(nameLens)('Hiro')(myObj) // { name: 'Hiro', bird: 'wood pigeon' }
  // overName(x => x + x)(myObj) // { name: 'JFJF', bird: 'wood pigeon' }
);