# flagged-respawn [](http://travis-ci.org/js-cli/js-flagged-respawn)
> A tool for respawning node binaries when special flags are present.
[](https://nodei.co/npm/flagged-respawn/)
## What is it?
Say you wrote a command line tool that runs arbitrary javascript (e.g. task runner, test framework, etc). For the sake of discussion, let's pretend it's a testing harness you've named `testify`.
Everything is going splendidly until one day you decide to test some code that relies on a feature behind a v8 flag in node (`--harmony`, for example). Without much thought, you run `testify --harmony spec tests.js`.
It doesn't work. After digging around for a bit, you realize this produces a [`process.argv`](http://nodejs.org/docs/latest/api/process.html#process_process_argv) of:
`['node', '/usr/local/bin/test', '--harmony', 'spec', 'tests.js']`
Crap. The `--harmony` flag is in the wrong place! It should be applied to the **node** command, not our binary. What we actually wanted was this:
`['node', '--harmony', '/usr/local/bin/test', 'spec', 'tests.js']`
Flagged-respawn fixes this problem and handles all the edge cases respawning creates, such as:
- Providing a method to determine if a respawn is needed.
- Piping stderr/stdout from the child into the parent.
- Making the parent process exit with the same code as the child.
- If the child is killed, making the parent exit with the same signal.
To see it in action, clone this repository and run `npm install` / `npm run respawn` / `npm run nospawn`.
## Sample Usage
```js
#!/usr/bin/env node
const flaggedRespawn = require('flagged-respawn');
// get a list of all possible v8 flags for the running version of node
const v8flags = require('v8flags').fetch();
flaggedRespawn(v8flags, process.argv, function (ready, child) {
if (ready) {
console.log('Running!');
// your cli code here
} else {
console.log('Special flags found, respawning.');
}
if (process.pid !== child.pid) {
console.log('Respawned to PID:', child.pid);
}
});
```
## API
### flaggedRespawn(flags, argv, [ forcedFlags, ] callback) : Void
Respawns the script itself when *argv* has special flag contained in *flags* and/or *forcedFlags* is not empty. Because members of *flags* and *forcedFlags* are passed to `node` command, each of them needs to be a node flag or a V8 flag.
#### Forbid respawning
If `--no-respawning` flag is given in *argv*, this function does not respawned even if *argv* contains members of flags or *forcedFlags* is not empty. (This flag is also used internally to prevent from respawning more than once).
#### Parameter:
| Parameter | Type | Description |
|:--------------|:------:|:----------------------------------------------------|
| *flags* | Array | An array of node flags and V8 flags which are available when present in *argv*. |
| *argv* | Array | Command line arguments to respawn. |
| *forcedFlags* | Array or String | An array of node flags or a string of a single flag and V8 flags for respawning forcely. |
| *callback* | function | A called function when not respawning or after respawned. |
* **callback(ready, proc, argv) : Void**
*callback* function is called both when respawned or not, and it can be distinguished by callback's argument: *ready*. (*ready* indicates whether a process spawned its child process (false) or not (true), but it does not indicate whether a process is a spawned child process or not. *ready* for a spawned child process is true.)
*argv* is an array of command line arguments which is respawned (when *ready* is false) or is passed current process except flags within *flags* and `--no-respawning` (when *ready* is true).
**Parameter:**
| Parameter | Type | Description |
|:----------|:-------:|:--------------------------|
| *ready* | boolean | True, if not respawning and is ready to execute main function. |
| *proc* | object | Child process object if respawned, otherwise current process object. |
| *argv* | Array | An array of command line arguments. |
## Release History
* 2017-12-16 - v1.0.0 - Force/Forbid respawn, Improved API & testing
* 2016-03-22 - v0.3.2 - fix issue with v8 flags values being dropped
* 2014-09-12 - v0.3.1 - use `{ stdio: 'inherit' }` for spawn to maintain colors
* 2014-09-11 - v0.3.0 - for real this time
* 2014-09-11 - v0.2.0 - cleanup
* 2014-09-04 - v0.1.1 - initial release