Gattai Merge

High-performance deep merge utility with structural sharing. Supports circular ref and complex built-in types.

Fast (copy-on-write, minimal cloning)
Structural sharing (immutable-friendly)
Supports circular ref
Handles Map, Set, Array, TypedArray, Date, RegExp, etc.
Customizable array merge functions
Optional descriptor preservation

Install

npm i gattai-merge

// npm
import gattaiMerge from 'gattai-merge';

// CDN
import gattaiMerge from 'https://cdn.jsdelivr.net/npm/gattai-merge/+esm';

Usage

import gattaiMerge from 'gattai-merge';

const a = { foo: 1, nested: { x: 1 } };
const b = { bar: 2, nested: { x: 1 } };

const result = gattaiMerge(a, b);

console.log(result);
// { foo: 1, bar: 2, nested: { x: 1 } }

API

gattaiMerge(target, ...sources)
gattaiMerge(target, ...sources, options)

Options

{
  arrays: 'replace';
  nullish: 'loose';
  preserveDescriptors: false;
  strictDescriptors: false;
}

arrays

'replace': replace target array (shallow copy)
'concat': concatenate arrays
'merge': deep merge by index
ArrayMergeFunction: custom array merge function (advanced usage)

nullish

'loose': keep target value if source is nullish
'strict': overwrite target value if source is nullish
'throw': throw TypeError if source is nullish

preserveDescriptors

false: use standard merge (faster, ignores property descriptors)
true: preserve property descriptors (getters/setters, etc.)

strictDescriptors

false: skip incompatible descriptors
true: throw if descriptor cannot be merged (e.g. non-configurable or non-writable)

🪄 ArrayMergeFunction

(target, source, {
  merge: (target, source) => {};
  clone: (node) => {};
}) => {};

Examples

Array

gattaiMerge([1, 2], [3, 4]);
// => [3, 4]

gattaiMerge([1, 2], [3, 4], { arrays: 'concat' });
// => [1, 2, 3, 4]

gattaiMerge([{ a: 1 }], [{ b: 2 }], { arrays: 'merge' });
// => [{ a: 1, b: 2 }]

Custom array merge function

gattaiMerge(
  [{ id: 1, value: 'A' }],
  [{ id: 1, value: 'B' }, { id: 2, value: 'C' }],
  {
    // merge items by id
    arrays: (target, source, { merge, clone }) => {
      const map = new Map();

      for (const item of target) {
        map.set(item.id, item);
      }

      for (const item of source) {
        if (map.has(item.id)) {
          map.set(item.id, merge(map.get(item.id), item));
        } else {
          map.set(item.id, clone(item));
        }
      }

      return Array.from(map.values());
    },
  }
);
// => [{ id: 1, value: 'B' }, { id: 2, value: 'C' }]

Map / Set

gattaiMerge(
  new Map([['a', 1]]),
  new Map([['b', 2]])
);
// => Map { 'a' => 1, 'b' => 2 }

Caution

Gattai Merge is optimized for performance using structural sharing (copy-on-write).

Objects are only cloned when a change is actually required.

What this implies

If no changes occur during merging, the original target object is returned as-is:

const a = { x: 1 };
const b = { x: 1 };

const result = gattaiMerge(a, b);

result === a; // true

Important

Because the same ref may be returned, mutating the result can also mutate the original input:

result.x = 2;

console.log(a.x); // 2 (mutated!)

When does this happen?

When merging produces no effective changes
When merging Map, Set, or nested structures with identical values
When structural sharing is preserved for performance

How to avoid this

1. Force a new object

const result = gattaiMerge({}, a, b);

2. Defensive cloning

const result = gattaiMerge(a, b);
const safe = result === a ? { ...result } : result;

Design note

This behavior is intentional and aligns with libraries like Immer, prioritizing performance by avoiding unnecessary cloning.

If you require strict immutability guarantees, consider wrapping or extending the API to always return a new object.

Performance

Avoids unnecessary cloning
Only clones changed branches
Comparable or faster than typical deep merge libraries in real-world scenarios