Your pug file won't have scope of randomString unless it is passed in when you call render() in your file that is calling it (such as your controller).

e.g.

this.render("[pugFilename]", {
    randomString = require("randomstring") // whatever package name you're using
}

Personally, I prefer doing any non-view stuff outside the view and in the script that is requesting the view to be rendered, where I can.

The syntax in Pug can start to look very messy otherwise and become difficult to follow.

You can switch out the code above with the code in your question and it should work fine, although I'd recommend changing your variable name (or key) to something more meaningful.

e.g

this.render("[pugFilename]", {
    randomStr: randomString({ length: 20 })
});

Update for phpStorm File Watcher

Firstly, install pug-cli and random-string locally,

npm install pug-cli random-string --save

Then setup File Watcher,

Original answer

Set and require random-string as a local variable, then you can access it in the templates. For example,

template.pug

.site
   .title
       - var string = randomString({length: 20});
       | #{string}

compile it using pug

const pug = require('pug');

// Compile template.pug, and render a set of data
console.log(pug.renderFile('template.pug', {
  randomString: require('random-string')
}));
//-> <div class="site"><div class="title">o0rGsvgEEOHrxj7niivt</div></div>

If you are using pug-cli, install random-string in the same scope of pug-cli, and specify options through a string or a file.

pug -O "{randomString: require('random-string')}" template.pug