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Merge objects/arrays declaratively.

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Use cases

This is intended for cases where objects/arrays manipulation in JavaScript is not available.

For example, a library where shared configuration files can be extended.

extend: my-shared-config

# Deep merge
log:
  verbosity: silent
  # Shallow merge
  provider:
    _merge: shallow
    name: redis
    type: local
  # Delete properties
  output:
    _merge: delete

rules:
  # Update arrays deeply
  1:
    level: silent
  # Append arrays
  '-0':
    name: appendedRule

Or a server receiving network patch requests.

PATCH /pets/0
{
  // Deep merge
  "capabilities": { "play": true },
  // Shallow merge
  "title": { "name": "felix", "_merge": "shallow" },
  // Append arrays
  "toys": { "-0": "newToy" }
}

Examples

Objects

Deep merge

import declarativeMerge from 'declarative-merge'

declarativeMerge({ a: 1, b: { c: 2 }, d: 3 }, { a: 10, b: { e: 20 } })
// { a: 10, b: { c: 2, e: 20 }, d: 3 }

Shallow merge

declarativeMerge(
  { a: 1, b: { c: 2 }, d: 3 },
  { a: 10, b: { e: 20 }, _merge: 'shallow' },
)
// { a: 10, b: { e: 20 }, d: 3 }

No merge

declarativeMerge(
  { a: 1, b: { c: 2 }, d: 3 },
  { a: 10, b: { e: 20 }, _merge: 'set' },
)
// { a: 10, b: { e: 20 } }

Nesting

// `_merge` can be specified in nested objects
declarativeMerge(
  { a: 1, b: { c: 2 }, d: 3 },
  { a: 10, b: { e: 20, _merge: 'set' } },
)
// { a: 10, b: { e: 20 }, d: 3 }

declarativeMerge(
  { a: 1, b: { c: 2 }, d: 3 },
  { a: 10, b: { e: 20, _merge: 'deep' }, _merge: 'set' },
)
// { a: 10, b: { c: 2, e: 20 } }

Delete

declarativeMerge(
  { a: 1, b: { c: 2 }, d: 3 },
  { a: 10, b: { e: 20, _merge: 'delete' } },
)
// { a: 10, d: 3 }

Arrays

Update

// By default, arrays override each other
declarativeMerge({ one: ['a', 'b', 'c'] }, { one: ['X', 'Y'] }) // { one: ['X', 'Y'] }

// They can be updated instead using an object where the keys are the array
// indices (before any updates).
declarativeMerge(
  { one: ['a', 'b', 'c'], two: 2 },
  { one: { 1: 'X' }, three: 3 },
)
// { one: ['a', 'X', 'c'], two: 2, three: 3 }

// This works on top-level arrays too
declarativeMerge(['a', 'b', 'c'], { 1: 'X', 2: 'Y' }) // ['a', 'X', 'Y']

Merge

// If the new array items are objects, they are merged
declarativeMerge(
  [{ id: 'a' }, { id: 'b', value: { name: 'Ann' } }, { id: 'c' }],
  { 1: { value: { color: 'red' } } },
)
// [{ id: 'a' }, { id: 'b', value: { name: 'Ann', color: 'red' } }, { id: 'c' }]

declarativeMerge(
  [{ id: 'a' }, { id: 'b', value: { name: 'Ann' } }, { id: 'c' }],
  { 1: { value: { color: 'red' }, _merge: 'shallow' } },
)
// [{ id: 'a' }, { id: 'b', value: { color: 'red' } }, { id: 'c' }]

Indices

declarativeMerge(['a', 'b', 'c'], { '*': 'X' }) // ['X', 'X', 'X']
declarativeMerge(['a', 'b', 'c'], { '-1': 'X' }) // ['a', 'b', 'X']
declarativeMerge(['a', 'b', 'c'], { 4: 'X' }) // ['a', 'b', 'c', undefined, 'X']

Add

// Array of items can be used
declarativeMerge(['a', 'b', 'c'], { 1: ['X', 'Y'] }) // ['a', 'X', 'Y', 'c']
declarativeMerge(['a', 'b', 'c'], { 1: ['X'] }) // ['a', 'X', 'c']
declarativeMerge(['a', 'b', 'c'], { 1: [['X']] }) // ['a', ['X'], 'c']

Insert

// If the key ends with +, items are prepended, not replaced
declarativeMerge(['a', 'b', 'c'], { '1+': 'X' }) // ['a', 'X', 'b', 'c']

Append

declarativeMerge(['a', 'b', 'c'], { '-0': 'X' }) // ['a', 'b', 'c', 'X']
declarativeMerge(['a', 'b', 'c'], { '-0': ['X', 'Y'] }) // ['a', 'b', 'c', 'X', 'Y']

Prepend

declarativeMerge(['a', 'b', 'c'], { '0+': ['X', 'Y'] }) // ['X', 'Y', 'a', 'b', 'c']

Delete

declarativeMerge(['a', 'b', 'c'], { 1: [] }) // ['a', 'c']

Set

declarativeMerge({}, { one: { 0: 'X', 2: 'Z' } }) // { one: ['X', undefined, 'Z'] }
declarativeMerge({ one: true }, { one: { 0: 'X', 2: 'Z' } }) // { one: ['X', undefined, 'Z'] }

Install

npm install declarative-merge

This package works in both Node.js >=18.18.0 and browsers.

This is an ES module. It must be loaded using an import or import() statement, not require(). If TypeScript is used, it must be configured to output ES modules, not CommonJS.

API

declarativeMerge(firstValue, secondValue, options?)

firstValue any
secondValue any
options Options
Return value: any

Merge firstValue and secondValue.

Merge mode

Any object can change the merge mode using a _merge property with value "deep" (default), "shallow", "set" or "delete".

Arrays can be merged using objects where the keys are the array indices. Items can be updated, merged, added, inserted, appended, prepended, deleted or set.

The _merge property and array updates objects can only be used in secondValue. They are left as is in firstValue.

Cloning

firstValue and secondValue are not modified. Plain objects and arrays are deeply cloned.

Inherited and non-enumerable properties are ignored.

Options

Options are an optional object.

key

Type: string | symbol
Default: "_merge"

Name of the property used to specify the merge mode.

declarativeMerge({ a: 1 }, { b: 2, _mergeMode: 'set' }, { key: '_mergeMode' }) // { b: 2 }

Symbols can be useful to prevent injections when the input is user-provided.

const mergeMode = Symbol('mergeMode')
declarativeMerge({ a: 1 }, { b: 2, [mergeMode]: 'set' }, { key: mergeMode }) // { b: 2 }

Related projects

Support

For any question, don't hesitate to submit an issue on GitHub.

Everyone is welcome regardless of personal background. We enforce a Code of conduct in order to promote a positive and inclusive environment.

Contributing

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If the documentation is unclear or has a typo, please click on the page's Edit button (pencil icon) and suggest a correction.

If you would like to help us fix a bug or add a new feature, please check our guidelines. Pull requests are welcome!