defu/README.md

# 🇩 defu

> Recursively assign default properties. Lightweight and Fast!

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## Install

Install package:

```bash
yarn add defu
# or
npm install defu
```

## Usage

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

Leftmost arguments have more priority when assigning defaults.

### Arguments

- **object (Object):** The destination object.
- **source (Object):** The source object.

```js
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

```js
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.

```js

defu.fn({
  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.

```js

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

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

### Remarks

- `object` and `defaults` are not modified
- `null` values are skipped same as [defaults-deep](https://www.npmjs.com/package/defaults-deep). Please use either [omit-deep](http://npmjs.com/package/omit-deep) or [lodash.defaultsdeep](https://www.npmjs.com/package/lodash.defaultsdeep) if you need to preserve.
- 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)
```js
console.log(defu({ array: ['b', 'c'] }, { array: ['a'] }))
// => { array: ['a', 'b', 'c']}
```

## License

MIT. Made with 💖

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