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# define-lazy-prop [](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)
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