rollup-merge-config

0.0.12 verified Mon Apr 27 auth: no javascript maintenance

Utility to merge multiple Rollup configuration objects with support for custom array and object merging strategies. Current stable version: 0.0.12 (released 2019). Low release cadence (no updates since 2019). Key differentiator: provides array concatenation and object merging specifically tailored for Rollup configs, with optional customizer functions to override default behavior for specific keys. Unlike generic merge libraries (e.g., lodash.merge), it handles Rollup's plugin arrays and nested config structures by default.

Common errors

error Uncaught TypeError: merge is not a function ↓

cause Using default import from CommonJS require where merge is a named export.

fix

Use import { merge } from 'rollup-merge-config' instead of const merge = require('rollup-merge-config').merge

error TypeError: config.push is not a function ↓

cause Misunderstanding that merge can handle arrays of configs like array spread; but merge expects multiple arguments, not an array.

fix

Use merge(config1, config2) or merge(...arrayOfConfigs) with spread, not merge(arrayOfConfigs).

Warnings

gotcha Arrays are concatenated by default, not replaced. This may cause duplicate plugins if the same plugin appears in multiple configs. ↓

fix Use a custom arrayCustomizer to deduplicate, e.g., arrayCustomizer: (a, b, key) => key === 'plugins' ? [...new Set([...a, ...b])] : undefined

gotcha Properties with undefined values are not merged and are skipped. If a config explicitly sets a property to undefined, the previous value remains. ↓

fix Ensure no config sets a property to undefined if you want it removed; use null instead.

gotcha Deep nested objects are merged recursively, but only plain objects. Values like instances of classes or functions are replaced, not merged. ↓

fix If you need to merge non-plain objects, implement a custom objectCustomizer.

deprecated Package has not been updated since 2019; no TypeScript definitions, no Rollup 3+ support. ↓

fix Consider using @rollup/pluginutils mergeOptions or manually merging configs.

Install

npm install rollup-merge-config

yarn add rollup-merge-config

pnpm add rollup-merge-config

Imports

merge
wrong
```
const merge = require('rollup-merge-config')
```
correct
```
import { merge } from 'rollup-merge-config'
```
ESM only; CommonJS require will return the module object, not the merge function directly.

default

wrong

const { merge } = require('rollup-merge-config')

correct

import rollupMergeConfig from 'rollup-merge-config'

Default export is an object containing merge and extend. Named exports are preferred.

extend
```
import { extend } from 'rollup-merge-config'
```
Identical to merge function; provided for semantic preference.

Quickstart

Shows basic merging of multiple Rollup config objects with default array concatenation and later-key override.

import { merge } from 'rollup-merge-config';

const baseConfig = {
  input: 'src/index.js',
  output: { file: 'dist/bundle.js', format: 'cjs' },
  plugins: []
};

const clientConfig = {
  output: { file: 'dist/client.js', format: 'iife' },
  plugins: []
};

const serverConfig = {
  output: { file: 'dist/server.js', format: 'cjs' },
  plugins: []
};

// Merge multiple configs; later keys override earlier ones.
// Arrays (like plugins) are concatenated.
const merged = merge(baseConfig, clientConfig, serverConfig);
console.log(merged);
// Output: {
//   input: 'src/index.js',
//   output: { file: 'dist/server.js', format: 'cjs' },
//   plugins: []
// }