Pass argument from script to gulp task

Question

I have package.json scripts with the following structure:

"scripts": {
    "watch:build": "tsc --watch",
    "watch:server": "nodemon ./src/app.js --watch 'app'",
    "build": "tsc && gulp do_something",
    "start": "npm-run-all clean build --parallel watch:build", 
    "watch:server --print-label"
}

I would like to start the application as npm run start with_argument and pass it to build script, to perform actions in the gulp task based on that argument.

I read a lot of tutorial and how to articles, but without result. It is possible somehow to pass argument from one script to another(which starts a gulp task).

Thanks in advance!

Answer 1

npm-run-all provides a its own custom mechanism for handling arguments by utilizing placeholders in npm-scripts, as stated in the Argument Placeholders section of its documentation found here.

npm-script:

Given your current npm-script named start you'll need to redefine it as follows:

"scripts": {
  ...
  "start": "npm-run-all clean \"build -- {@}\" --parallel watch:build --"
  ...
}

Notes:

• -- {@} must be added after build.¹
• build -- {@} must be wrapped in escaped double quotes \"...\"
• -- must also be added after last script invocation, namely: watch:build

---

gulpfile.js

To obtain the arguments passed via the CLI inside your gulpfile.js you''ll need to utilize nodes process.argv

For the purpose of demonstration lets say our gulpfile.js is as follows:

var gulp = require('gulp');
var args = process.argv.splice(3, process.argv.length - 3);

gulp.task('doSomething', function() {

  // For testing purposes...
  if (args.indexOf('--foo') > -1) {
    console.log('--foo was passed via the CLI.')
  }

  if (args.indexOf('--quux') > -1) {
    console.log('--quux was passed via the CLI.')
  }
});

Notes:

1. The first three items in nodes process.argv are:

   • The path to the executable running the JavaScript file.
   • The path of the JavaScript file being executed.
   • The name of the gulp task, i.e. doSomething

2. However, we're only interested in the elements from the fourth item in the array onwards - as these will be the arguments passed via the CLI. The line which reads:

   var args = process.argv.splice(3, process.argv.length - 3);
   

   creates an args variable and assigns an Array containing each argument passed via the CLI, i.e. we omit the first three aforementioned items in point 1 above using the Arrays splice() method.

---

Running your start script:

You invoke your start script via your CLI as follows:

$ npm start -- --foo --quux

Note You must provide the -- which precedes npm start before providing your own arguments.

---

Output:

• Using the contrived gulpfile.js above, in combination with your current scripts defined in your package.json, and of course the necessary changes made to your start script. When you run:

  $ npm start -- --foo --quux
  

  you will see the following printed to the console:

  --foo was passed via the CLI.

  --quux was passed via the CLI.

• Running:

  $ npm start -- --quux
  

  you will see just the following printed to the console:

  --quux was passed via the CLI.

• And of course, running:

  $ npm start
  

  Does not print either of the messages defined in gulpfile.js.

---

_Footnotes:

1 _{-- {@} can be replaced with -- {1} if you only intend to pass one argument. However, -- {@} handles multiple arguments, so it's fine to use it for one argument too.}

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