# define-lazy-prop [![Build Status](https://travis-ci.org/sindresorhus/define-lazy-prop.svg?branch=master)](https://travis-ci.org/sindresorhus/define-lazy-prop)

> Define a [lazily evaluated](https://en.wikipedia.org/wiki/Lazy_evaluation) property on an object

Useful when the value of a property is expensive to generate, so you want to delay the computation until the property is needed. For example, improving startup performance by deferring nonessential operations.


## Install

```
$ npm install define-lazy-prop
```


## Usage

```js
const defineLazyProp = require('define-lazy-prop');

const unicorn = {
	// …
};

defineLazyProp(unicorn, 'rainbow', () => expensiveComputation());

app.on('user-action', () => {
	doSomething(unicorn.rainbow);
});
```


## API

### defineLazyProp(object, propertyName, fn)

#### object

Type: `Object`

Object to add property to.

#### propertyName

Type: `string`

Name of the property to add.

#### fn

Type: `Function`

Called the first time `propertyName` is accessed. Expected to return a value.


## Related

- [lazy-value](https://github.com/sindresorhus/lazy-value) - Create a lazily evaluated value
- [import-lazy](https://github.com/sindresorhus/import-lazy) - Import a module lazily
- [p-lazy](https://github.com/sindresorhus/p-lazy) - Create a lazy promise


## License

MIT © [Sindre Sorhus](https://sindresorhus.com)