A simple function to mutate basic JavaScript variables in an immutable fashion

This package provides a getWithMutations(oldValue, newValue) function that returns the oldValue when the values are equal, or returns the newValue when the values are different.

It operates recursively on objects or arrays, preserving nested value equality whenever possible.

Installation

Install the plugin with npm:

$ npm install --save with-mutations

Basic Usage

Mutating two equal values:

import getWithMutations from 'with-mutations';

const oldValue = { a: 1 };
const newValue = { a: 1 };
const result = getWithMutations(oldValue, newValue);

console.log(result === oldValue); // true
console.log(result === newValue); // false

Mutating two different values with some overlap:

import getWithMutations from 'with-mutations';

const oldValue = [ { a: 1 }, { b: 1 } ];
const newValue = [ { a: 1 }, { b: 2 } ];
const result = getWithMutations(oldValue, newValue);

console.log(result === oldValue); // false
console.log(result === newValue); // false

console.log(result[0] === oldValue[0]); // true
console.log(result[1] === oldValue[1]); // false

console.log(result[0] === newValue[0]); // false
console.log(result[1] === newValue[1]); // true

Custom mutatators

The getWithMutations function takes an optional third argument which is the custom mutator.

The customMutator should be a function which takes the oldValue and newValue as arguments and returns the result.

Note that the custom mutator is only executed if all of the following are true:

• Neither of the values are null or undefined
• The values are not identical references or primitive values
• The values are not both arrays or objects

import getWithMutations from 'with-mutations';

const oldValue = () => { };
const newValue = () => { };
oldValue.prop = 'a';
newValue.prop = 'a';

const customMutator = (oldValue, newValue) => oldValue.prop === newValue.prop ? oldValue : newValue;

const result = getWithMutations(oldValue, newValue, customMutator)

console.log(result === oldValue); // true
console.log(result === newValue); // false