> For the complete documentation index, see [llms.txt](https://stampit.js.org/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://stampit.js.org/essentials/specification/merging-algorithm.md).

# Merging algorithm

When composing stamps you actually merge their [descriptors](/essentials/what-is-a-stamp.md) (aka metadata).

The `Object.assign` is used for these descriptor properties:

* [methods](/api/methods.md)
* [properties](/api/properties.md)
* [propertyDescriptors](/api/property-descriptors.md)
* [staticProperties](/api/static-properties.md)
* [staticPropertyDescriptors](/api/static-property-descriptors.md)
* [configuration](/api/configuration.md)

The special deep merging algorithm (see below) is used for these descriptor properties:

* [deepProperties](/api/deep-properties.md)
* [staticDeepProperties](/api/static-deep-properties.md)
* [deepConfiguration](/api/deep-configuration.md)

The array concatenation and deduplication (see below) is used for these descriptor properties:

* [initializers](/api/initializers.md)
* [composers](/api/composers.md)

## Array concatenation and deduplication

This is how initializers and composers metadata gets merged.

```javascript
const _ = require('lodash')

descriptor.initializers = _.uniq(_.concat(descriptor1.initializers, descriptor2.initializers))
descriptor.composers = _.uniq(_.concat(descriptor1.composers, descriptor2.composers))
```

See the [exact line](https://github.com/stampit-org/stamp-specification/blob/8a5448806da286b2f840c33cb618ddbcdb40182d/compose.js#L162) of the reference implementation source code.

## Deep merging algorithm

The [stamp specification](/essentials/specification.md) standardized the deep merging algorithm to, basically, this:

* Plain objects are (recursively) deep merged, including ES6 Symbol keys, ES5 getters and setters.
* **Arrays are concatenated**.
* Functions, Symbols, RegExp, etc. values are copied by reference.
* The last object type overwrites previous object type.

```javascript
const MyStamp1 = stampit()

const deepObject1 = {
  [Symbol('foo')]: { one: 'first' },                 // this plain object will be deep merged

  array: [0, 'bar', () => {}, { obj: 'my object' }], // this array will be concatenated with

  func: MyStamp1,                                    // this stamp will be replaced with another stamp

  something: [42],                                   // this array will be replaced with an object

  oldKey: 'some value'
}

const MyStamp2 = stampit()

const deepObject2 = {
  [Symbol('foo')]: { two: 'second' },

  array: [0, 'bar', { another: 'object' }],

  func: MyStamp2,

  something: { [0]: 42 },

  newKey: 'some value'
}
```

The merged result of the two objects above will be this:

```javascript
const deepResult = {
  [Symbol('foo')]: { one: 'first', two: 'second' }, // contains properties from both objects

  array: [0, 'bar', () => {}, { obj: 'my object' }, 0, 'bar', { another: 'object' }], // both arrays are here

  func: MyStamp2,                                   // Stamp2 replaced the Stamp1

  something: { [0]: 42 },                           // the array was replaced with the object

  oldKey: 'some value',                             // these two were simply carried across
  newKey: 'some value'
}
```

See the [exact line](https://github.com/stampit-org/stamp-specification/blob/8a5448806da286b2f840c33cb618ddbcdb40182d/compose.js#L70) of the reference implementation source code.