immutable-setter

Motivation

There is a lot written about advantages of using immutable data structures in you code. The problem with JavaScrip that it is require en extra effort, "syntactical noise" and discipline because immutability is not "enforced" by the league. Especially if need to update deeply nested object structure.

To help with this problems library like Immutable.js can be used. Immutable.js uses special version containers like Set, Map etc. and special version of object class called Record to enforce immutability in your code.

A different approach is taken by popular application state management library Redux that treat application state as immutable object. Redux uses pure functions (called reducers). Reducer function accept original parts of the State object and modification parameters (actions) and return the new State Part with changes applied. Redux "updates" the application State by "cascading" calls to reducers with corresponding State Parts and "reassembling" the State in immutable fashion.

Idea of immutable-setter (This library) is to modify, retrieve and delete deeply nested parts of regular JavaScript object structure in immutable fusion. (Functions: setIn, getIn, deleteIn). Immutable-setter functions do not modify original object, but return new object that contains modification. Immutable-setter preserves Structural Sharing - only modified parts of the object structure are copied.

Immutable-setter uses concept of KeyPath to specify target parts of the object structure. KeyPath is an array of keys and indexes that describe the path to the target part of the object structure.

Immutable-setter gracefully handles situations when KeyPath points to the missing parts of the Object Structure:
setIn - missing parts will be generated. Immutable-setter infers type (Object vs. Array) to be generated based on KeyPath part pointing to the object's children. If key is a string then Object class instance is created, if key is and Integer or undefined* - Array instance is created.
getIn - if any part of the object structure is missing it will return undefined*
* deleteIn - will do nothing

Please see Examples bellow and Unit tests for more details.

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``js

//update existing property deep in the object structure 'bar' -> 'baz'
constant original1 = {
a: 'foo',
b: [{
c: 'bar'
}]
}
constant result1 = setIn(original1, ['b', 0, 'c'], 'baz')
> result1: {
a: 'foo',
b: [{
c: 'baz'
}]
}

//create sub object and set property
constant original2 = {
a: 'foo'
}
constant result2 = setIn(original2, ['b', 'c'], 'bar')
> result2: {
a: 'foo',
b: {
c: 'bar'
}
}

//append object to the not existing yet array
constant original3 = {
a: 'foo'
}
constant result3 = setIn(original3, ['b', undefined], {c: 'bar'})
> result3: {
a: 'foo',
b: [{
c: 'bar'
}]
}

//create new object with the value and append it to the array that not yet exists
constant original4 = {
a: 'foo'
}
constant result4 = setIn(original4, ['b', undefined, 'c'], 'bar')
> result4: {
a:'foo',
b: [{
c:'bar'
}]
}
`
For more examples check the test file


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`
setIn(object: Object, keyPath: Array, value: Any) => Object
`

* object - plain javascript object. It will be treated as immutable.
* keyPath - array of keys and indexes specifying path to the property to be set in the objects hierarchy.
Can contain undefined keys indicating that value should be pushed to the end of the array.
* value - new value to be set

* return - new object with new objects created along the keyPath

`
deleteIn(object: Object, keyPath: Array) => Object
`

* object - plain javascript object. It will be treated as immutable.
* keyPath - array of keys and indexes specifying path to the property to be deleted in the objects hierarchy.

* return - new object with new objects created along the keyPath if specified keyPath value was deleted.
It returns original objects if value along keyPath was not found

`
getIn(object: Object, keyPath: Array) => Any|undefined
`
* object - plain javascript object. It will be treated as immutable.
* keyPath - array of keys and indexes specifying path of the property to read.

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If Immutable.js is used - it provides a similar API
for the Map object, except it requires all parts of the keyPath to be defined even if it is appending to the array (index of the new element).

The React Immutability Helpers library has more functionality for manipulating immutable values but I found the syntax a bit 'heavy' to read. Also, it does not support appending object to the non existing array. For example:
`js
const newObj = update({}, {a: {b: {$push:[{c: 'foo'}]}}});
//This fails!
expect(newObj.a.b[0].c).to.be.equal("foo");

`

For non-immutable use cases deep-get-set can be used.

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* npm install immutable-setter
`js
include { setIn, deleteIn, getIn } from 'immutable-setter'
``

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* clone the repository
* cd immutable-setter
* npm test - this will build the library, generate the lib directory, and will run tests against the build.

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MIT