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StateLens is a tiny (11 LOC) lib that uses the Ramda library, particularly the lens functions, to provide a lens structure that is useful for state manipulations.

It was initially written to support large React applications without the need for a complex state container.


Install via npm

npm install statelens


StateLens provides a single constructor function which takes three parameters, a setter, getter and initial focus.


var statelens = require('statelens');

var state = {a: {b: 0}};

var get = function() { return state; } var set = function(upd) { state = upd(state); }

var lens = statelens(set, get, ['a']);

// immutable read = 10; console.log( // 0

lens.narrow(['b']).set(10) console.log( // 10

lens.narrow(['b']).modify(function(n) { return n + 1; }); console.log( // 11


(State -> State) -> void

A function which accepts a state transformation function as it's sole parameter, and it should execute the passed transformation by passing in the current state.


() -> State

A zero parameter function that returns the current state.



An array of strings and numbers indicating the path in the state structure on which the initial focus is set. For example with a state of {a: [{foo: 'bar'}]}, a focus of ['a', 0, 'foo'] will be set on the location of the value 'bar'.


The provided constructor returns an object with four functions on it.


(A) -> void

Replaces the focus of the current lens with the provided value, and passes the resulting state to the lenses Setter.


(Focus -> A) -> void

Takes a function from focused portion of the current state to a new value.

Passes the focused portion of the current state to the provided function, replaces the focus of the current state with the result, and passes the resulting state to the lenses Setter.


() -> Focus

Returns a deep copy of the focused portion of the current state.


([number|string]) -> StateLens

Takes a new focus parameter, and returns a new lens focused on the concatentation of the original lenses path with the newly provided path.