merge

Merge immutable model state (or any data structure, really), preserving references to unchanged nodes, such that the === operation can be used to determine where state has changed. (Useful when dealing with immutable data models such as React+Redux, where === is used for exactly this purpose.)

Specifically, given

• Two JSON data structures, before and after
• state = merge(before, after)

Then:

• assert.deepEqual(state, after) always passes

And for any path, [X], to an Object or Array within state:

• state[X] === before[X] where assert.deepEqual(before[X], state[X]) passes
• state[X] === after[X] where no part of after[X] is equal to before[X]
• state[X] === (new Object/Array) when some, but not all state w/in [X] has changed

Installation

npm i @broofa/merge

Example

const assert = require('assert');
const merge = require('@broofa/merge');

const before = {
  a: 'hello',
  b: 123,
  c: {ca: ['zig'], cb: [{a:1}, {b:2}]},
};

const after = {
  a: 'world',
  b: 123,
  c: {ca: ['zig'], cb: [{a:99}, {b:2}]},
};

const state = merge(before, after);

assert.deepEqual(after, state); // Always true

// Where state HAS changed
state === before; // ⇨ false
state.c === before.c; // ⇨ false
state.c.cb === before.c.cb; // ⇨ false
state.c.cb[0] === before.c.cb[0]; // ⇨ false

// Where state HAS NOT changed
state.c.ca === before.c.ca; // ⇨ true
state.c.cb[1] === before.c.cb[1]; // ⇨ true

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