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🌊 defu

Assign default properties, recursively. Lightweight and Fast!

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Install package:

yarn add defu
# or
npm install defu


const options = defu (object, ...defaults)

Leftmost arguments have more priority when assigning defaults.


  • object (Object): The destination object.
  • source (Object): The source object.
const defu = require('defu')

console.log(defu({ 'a': { 'b': 2 } }, { 'a': { 'b': 1, 'c': 3 } }))
// => { a: { b: 2, c: 3 } }

Custom Merger

Sometimes default merging strategy is not desirable. Using defu.extend we can create a custom instance with different merging strategy.

This function accepts obj (source object), key and value (current value) and should return true if applied custom merging.

Example: Sum numbers instead of overriding

const ext = defu.extend((obj, key, value) => {
  if (typeof obj[key] === 'number' && typeof value === 'number') {
    obj[key] += val
    return true

ext({ cost: 15 }, { cost: 10 }) // { cost: 25 }

Function Merger

Using defu.fn, if user provided a function, it will be called with default value instead of merging.

I can be useful for default values manipulation.

Example: Filter some items from defaults (array) and add 20 to the count default value.

  ignore: (val) => val.filter(item => item !== 'dist'),
  count: (count) => count + 20
 }, {
   ignore: ['node_modules','dist'],
   count: 10
    ignore: ['node_modules'],
    count: 30

Note: if the default value is not defined, the function defined won't be called and kept as value.

Array Function Merger

defu.arrayFn is similar to defu.fn but only applies to array values defined in defaults.

Example: Filter some items from defaults (array) and add 20 to the count default value.

  ignore(val) => val.filter(i => i !== 'dist'),
  count: () => 20
 }, {
   ignore: [
   count: 10
    ignore: ['node_modules'],
    count: () => 20

Note: the function is called only if the value defined in defaults is an aray.


  • object and defaults are not modified
  • Nullish values (null and undefined) are skipped. Please use defaults-deep or omit-deep or lodash.defaultsdeep if you need to preserve or different behavior.
  • Assignment of __proto__ and constructor keys will be skipped to prevent security issues with object pollution.
  • Will concat array values (if default property is defined)
console.log(defu({ array: ['b', 'c'] }, { array: ['a'] }))
// => { array: ['a', 'b', 'c']}


MIT. Made with πŸ’–