Objektiv

Functional lenses in JavaScript

Objektiv (German) Lens, optics, objective

Basics

Objektiv.attr('field') Object attribute accessor. Objektiv.at(index) Array element accessor. lens.then(otherLens, ...) Lens composition can take many arguments. lens.traversal([filter]) Returns traversal, optionally filtered.

Regular lenses

Throw TypeError unless element has been found

const data = { array: [1, 2, 3] };
const firstOfSome = Objektiv.attr("array").at(0);

firstOfSome.get(data); //=> 1
firstOfSome.set(data, 10); //=> { array: [10, 2, 3] }
firstOfSome.map(data, x => x + 1); //=> { array: [2, 2, 3] }

Partial lenses

Skip missing element

const data = { array: [1, 2, 3] };
const firstOfMissing = Objektiv.partialAttr("missing").partialAt(0);

firstOfMissing.get(data); //=> undefined
firstOfMissing.set(data, 10); //=> { array: [1, 2, 3] }
firstOfMissing.map(data, x => x + 1); //=> { array: [1, 2, 3] }

Traversals

Traversals make working with series of data easy:

const data = { array: [{ x: 0, y: 9 }, { x: 1, y: 8 }, { x: 2, y: 7 }] };
const traversal = Objektiv.attr("array")
  .traversal()
  .attr("x");

traversal.get(data); //=> [0, 1, 2]
traversal.map(data, x => x + 1); //=> {array: [{x: 1, y:9}, {x: 2, y: 8}, {x: 3, y: 7}]}
traversal.set(data, 6); //=> {array: [{x: 6, y:9}, {x: 6, y: 8}, {x: 6, y: 7}]}

// Nested traversals
const data = {
  users: [{ id: 1, friends: ["Alice", "Bob"] }, { id: 2, friends: ["Sam"] }]
};
const traversal = Objektiv.attr("users")
  .traversal()
  .attr("friends")
  .traversal();

traversal.get(data); //=> [['Alice', 'Bob'], ['Sam']]
traversal.map(data, name => name.toUpperCase()); //=> {users: [{id: 1, friends: ['ALICE', 'BOB']}, {id: 2, friends: ['SAM']}]};

Filtered traversals

Array elements can be traversed by calling .traversal([predicate]) on a lens that references an array. You can pass predicate in traversal to filter elements of an array or chain .traversal().filter(predicate) function. Several .filter() functions can be chained one after another.

const data = { array: [{ x: 0, y: 9 }, { x: 1, y: 8 }, { x: 2, y: 7 }] };
const xGreaterThanOne = el => el.x > 1;
const traversal = Objektiv.attr("array")
  .traversal(xGreaterThanOne)
  .attr("x");
// Same as:
Objektiv.attr("array")
  .traversal()
  .filter(xGreaterThanOne)
  .attr("x");

traversal.get(data); //=> [2]
traversal.map(data, x => x + 1); //=> {array: [{x: 0, y:9}, {x: 1, y: 8}, {x: 3, y: 7}]}
traversal.set(data, 6); //=> {array: [{x: 0, y:9}, {x: 0, y: 8}, {x: 6, y: 7}]}

// Nested traversals
const data = {
  users: [{ id: 1, friends: ["Alice", "Bob"] }, { id: 2, friends: ["Sam"] }]
};
const hasTwoFriends = user => user.friends.length == 2;
const threeLetter = str => str.length === 3;

const traversal = Objektiv.attr("users")
  .traversal(hasTwoFriends)
  .attr("friends")
  .traversal(threeLetter);

traversal.get(data); //=> [['Bob']]
traversal.map(data, name => name.toUpperCase()); //=> {users: [{id: 1, friends: ['Alice', 'BOB']}, {id: 2, friends: ['Sam']}]};

Cursors

Objektiv also provides cursors which are lenses enclosed over data or root accessors. Cursor is a handy self-contained object to pass around. Cursors can be composed and traversed as regular lenses.

const data = {some: deep: 1};
const full = Objektiv.dataCursor(data);
const deepLens = Objektiv.attr('some').attr('deep');
const cursor = full.then(deepLens);

cursor.get() //=> 1
cursor.set(42)
cursor.get() //=> 42
full.get() //=> {some: deep: 42}
// All data are handled as immutable so original data is still:
data   //=> {some: deep: 1}

Cursor can notify about each change via callback:

const full = Objektiv.dataCursor(data, (newState, oldState) => {
  // ... deal with it
});

This way it can be used with ReactJS. Inside a component:

const that = this;
const full = Objektiv.dataCursor(this.state, function(state) {
  that.setState(state);
});
const childCursor = full.attr("child"); // ... and pass it to a child component

Outside of a root component:

const root = <Root ... />;
Objektiv.dataCursor({/** Initial data **/}, function (state) {
  root.setProps(state);
});
React.renderComponent(root, ...);