This project is part of the @thi.ng/umbrella monorepo.

This library provides immutable and mutable, optimized path-based accessors for nested, vanilla JS objects & arrays with structural sharing.

yarn add @thi.ng/paths

• @thi.ng/checks
• @thi.ng/errors

import * as paths from "@thi.ng/paths";

The getter(), setter() and updater() functions compile a lookup path like a.b.c into a function operating directly at the value the path points to in nested object. For getters, this essentially compiles to val = obj.a.b.c, with the important difference that the function returns undefined if any intermediate values along the lookup path are undefined (and doesn't throw an error).

The resulting setter function too accepts a single object (or array) to operate on and when called, immutably replaces the value at the given path, i.e. it produces a selective deep copy of obj up until given path. If any intermediate key is not present in the given object, it creates a plain empty object for that missing key and descends further along the path.

s = setter("a.b.c");
// or
s = setter(["a","b","c"]);

s({a: {b: {c: 23}}}, 24)
// {a: {b: {c: 24}}}

s({x: 23}, 24)
// { x: 23, a: { b: { c: 24 } } }

s(null, 24)
// { a: { b: { c: 24 } } }

Nested value updaters follow a similar pattern, but also take a user supplied function to apply to the existing value (incl. any other arguments passed):

inc = updater("a.b", (x) => x != null ? x + 1 : 1);

inc({a: {b: 10}});
// { a: { b: 11 } }
inc({});
// { a: { b: 1 } }

// with additional arguments
add = updater("a.b", (x, n) => x + n);

add({a: {b: 10}}, 13);
// { a: { b: 23 } }

In addition to these higher-order functions, the module also provides immediate-use wrappers: getIn(), setIn(), updateIn() and deleteIn(). These functions are using getter / setter internally, so have same behaviors.

state = {a: {b: {c: 23}}};

getIn(state, "a.b.c")
// 23

setIn(state, "a.b.c", 24)
// {a: {b: {c: 24}}}

// apply given function to path value
updateIn(state, "a.b.c", x => x + 1)
// {a: {b: {c: 24}}}

// immutably remove path key
deleteIn(state, "a.b.c.")
// {a: {b: {}}}

Only keys in the path will be updated, all other keys present in the given object retain their original/identical values to provide efficient structural sharing / re-use. This is the same behavior as in Clojure's immutable maps or those provided by ImmutableJS (albeit those implementation are completely different - they're using trees, we're using the ES6 spread op (for objects, slice() for arrays) and dynamic functional composition to produce the setter/updater).

s = setter("a.b.c");

// original
a = { x: { y: { z: 1 } }, u: { v: 2 } };
// updated version
b = s(a, 3);
// { x: { y: { z: 1 } }, u: { v: 2 }, a: { b: { c: 3 } } }

// verify anything under keys `x` & `u` is still identical
a.x === b.x // true
a.x.y === b.x.y // true
a.u === b.u; // true

mutator() is the mutable alternative to setter(). It returns a function, which when called, mutates given object / array at given path location and bails if any intermediate path values are non-indexable (only the very last path element can be missing in the actual object structure). If successful, returns original (mutated) object, else undefined. This function too provides optimized versions for path lengths <= 4.

As with setIn, mutIn is the immediate use mutator, i.e. the same as: mutator(path)(state, val).

mutIn({ a: { b: [10, 20] } }, "a.b.1", 23);
// or
mutIn({ a: { b: [10, 20] } }, ["a", "b", 1], 23);
// { a: { b: [ 10, 23 ] } }

// fails (because of missing path structure in target object)
mutIn({}, "a.b.c", 23);
// undefined

• Karsten Schmidt

© 2016 - 2018 Karsten Schmidt // Apache Software License 2.0