# hook-std [](https://travis-ci.org/sindresorhus/hook-std)
> Hook and modify stdout and stderr
## Install
```
$ npm install hook-std
```
## Usage
```js
const assert = require('assert');
const hookStd = require('hook-std');
(async () => {
const promise = hookStd.stdout(output => {
promise.unhook();
assert.strictEqual(output.trim(), 'unicorn');
});
console.log('unicorn');
await promise;
})();
```
You can also unhook using the second `transform` method parameter:
```js
(async () => {
const promise = hookStd.stdout((output, unhook) => {
unhook();
assert.strictEqual(output.trim(), 'unicorn');
});
console.log('unicorn');
await promise;
})();
```
## API
### hookStd([options], transform)
Hook streams in [streams option](#streams), by default stdout and stderr.
Returns a `Promise` with a `unhook()` method which, when called, unhooks both stdout and stderr and resolves the `Promise` with an empty result.
### hookStd.stdout([options], transform)
Hook stdout.
Returns a `Promise` with a `unhook()` method which, when called, resolves the `Promise` with an empty result.
### hookStd.stderr([options], transform)
Hook stderr.
Returns a `Promise` with a `unhook()` method which, when called, resolves the `Promise` with an empty result.
#### options
Type: `Object`
##### silent
Type: `boolean`
Default: `true`
Suppress stdout/stderr output.
##### once
Type: `boolean`
Default: `false`
Automatically unhooks after the first call.
##### streams
Type: `stream.Writable[]`
Default: `[process.stdout, process.stderr]`
[Writable streams](https://nodejs.org/api/stream.html#stream_writable_streams) to hook. This can be useful for libraries allowing users to configure a Writable Stream to write to.
#### transform
Type: `Function`
Receives stdout/stderr as the first argument and the unhook method as the second argument. Return a string to modify it. Optionally, when in silent mode, you may return a `boolean` to influence the return value of `.write(...)`.
## License
MIT © [Sindre Sorhus](https://sindresorhus.com)