The problem

You need to do some dynamic stuff, but don't want to do it at runtime. Or maybe you want to do stuff like read the filesystem to get a list of files and you can't do that in the browser.

This solution

This allows you to specify some code that runs in Node and whatever you module.exports in there will be swapped. For example:

const x = preval`module.exports = 1`

//      ↓ ↓ ↓ ↓ ↓ ↓

const x = 1

Or, more interestingly:

const x = preval`
  const fs = require('fs')
  const val = fs.readFileSync(__dirname + '/fixture1.md', 'utf8')
  module.exports = {
    val,
    getSplit: function(splitDelimiter) {
      return x.val.split(splitDelimiter)
    }
  }
`

//      ↓ ↓ ↓ ↓ ↓ ↓

const x = {
  val: '# fixture\n\nThis is some file thing...\n',
  getSplit: function getSplit(splitDelimiter) {
    return x.val.split(splitDelimiter)
  },
}

There's also preval.require('./something') and import x from /* preval */ './something' (which can both take some arguments) or add // @preval comment at the top of a file.

See more below.

Table of Contents

• Installation
• Usage
  • Template Tag
  • import comment
  • preval.require
  • preval file comment (// @preval)
• Exporting a function
• Configure with Babel
  • Via .babelrc (Recommended)
  • Via CLI
  • Via Node API
• Use with babel-plugin-macros
• Examples
• Notes
• FAQ
  • How is this different from prepack?
  • How is this different from webpack loaders?
• Inspiration
• Related Projects
• Other Solutions
• Issues
  • 🐛 Bugs
  • 💡 Feature Requests
• Contributors ✨
• LICENSE

Installation

This module is distributed via npm which is bundled with node and should be installed as one of your project's devDependencies:

npm install --save-dev babel-plugin-preval

Usage

Important notes:

1. All code run by preval is not run in a sandboxed environment
2. All code must run synchronously.
3. Code that is run by preval is not transpiled so it must run natively in the version of node you're running. (cannot use es modules).

You may like to watch this YouTube video to get an idea of what preval is and how it can be used.

Template Tag

Before:

const greeting = preval`
  const fs = require('fs')
  module.exports = fs.readFileSync(require.resolve('./greeting.txt'), 'utf8')
`

After (assuming greeting.txt contains the text: "Hello world!"):

const greeting = 'Hello world!'

preval can also handle some simple dynamic values as well:

Before:

const name = 'Bob Hope'
const person = preval`
  const [first, last] = require('./name-splitter')(${name})
  module.exports = {first, last}
`

After (assuming ./name-splitter is a function that splits a name into first/last):

const name = 'Bob Hope'
const person = {first: 'Bob', last: 'Hope'}

import comment

Before:

import fileList from /* preval */ './get-list-of-files'

After (depending on what ./get-list-of-files does, it might be something like):

const fileList = ['file1.md', 'file2.md', 'file3.md', 'file4.md']

You can also provide arguments which themselves are prevaled!

Before:

import fileList from /* preval(3) */ './get-list-of-files'

After (assuming ./get-list-of-files accepts an argument limiting how many files are retrieved:

const fileList = ['file1.md', 'file2.md', 'file3.md']

preval.require

Before:

const fileLastModifiedDate = preval.require('./get-last-modified-date')

After:

const fileLastModifiedDate = '2017-07-05'

And you can provide some simple dynamic arguments as well:

Before:

const fileLastModifiedDate = preval.require(
  './get-last-modified-date',
  '../../some-other-file.js',
)

After:

const fileLastModifiedDate = '2017-07-04'

preval file comment (// @preval)

Using the preval file comment will update a whole file to be evaluated down to an export.

Whereas the above usages (assignment/import/require) will only preval the scope of the assignment or file being imported.

Before:

// @preval

const id = require('./path/identity')
const one = require('./path/one')

const compose = (...fns) => fns.reduce((f, g) => a => f(g(a)))
const double = a => a * 2
const square = a => a * a

module.exports = compose(square, id, double)(one)

After:

module.exports = 4

Exporting a function

If you export a function from a module that you're prevaling (whether using preval.require or the import comment), then that function will be called and whatever is returned will be the prevaled value.

It's important to know this if you want to have the prevaled value itself be a function:

Example:

// example-module.js
const fn = message => `The message is: ${message}`
module.exports = () => fn

Usage of preval:

const theFn = preval.require('./example-module.js')

Generated code:

const theFn = message => `The message is: ${message}`

Configure with Babel

Via .babelrc (Recommended)

.babelrc

{
  "plugins": ["preval"]
}

Via CLI

babel --plugins preval script.js

Via Node API

require('babel-core').transform('code', {
  plugins: ['preval'],
})

Use with babel-plugin-macros

Once you've configured babel-plugin-macros you can import/require the preval macro at babel-plugin-preval/macro. For example:

import preval from 'babel-plugin-preval/macro'

const one = preval`module.exports = 1 + 2 - 1 - 1`

You could also use preval.macro if you'd prefer to type less 😀

Examples

• Mastodon saved 40kb (gzipped) using babel-plugin-preval
• glamorous-website uses preval.macro to determine Algolia options based on process.env.LOCALE. It also uses preval.macro to load an svg file as a string, base64 encode it, and use it as a background-url for an input element.
• Generate documentation for React components
• Serverless with webpack build serverless functions using webpack and Babel for development and production with preval to replace (possible sensible) content in code.
• Read files at build time (video)

Notes

If you use babel-plugin-transform-decorators-legacy, there is a conflict because both plugins must be placed at the top

Wrong:

{
  "plugins": ["preval", "transform-decorators-legacy"]
}

Ok:

{
  "plugins": ["preval", ["transform-decorators-legacy"]]
}

FAQ

How is this different from prepack?

prepack is intended to be run on your final bundle after you've run your webpack/etc magic on it. It does a TON of stuff, but the idea is that your code should work with or without prepack.

babel-plugin-preval is intended to let you write code that would not work otherwise. Doing things like reading something from the file system are not possible in the browser (or with prepack), but preval enables you to do this.

How is this different from webpack loaders?

This plugin was inspired by webpack's val-loader. The benefit of using this over that loader (or any other loader) is that it integrates with your existing babel pipeline. This is especially useful for the server where you're probably not bundling your code with webpack, but you may be using babel. (If you're not using either, configuring babel for this would be easier than configuring webpack for val-loader).

In addition, you can implement pretty much any webpack loader using babel-plugin-preval.

If you want to learn more, check webpack documentations about loaders.

Inspiration

I needed something like this for the glamorous website. I live-streamed developing the whole thing. If you're interested you can find the recording on my youtube channel (note, screen only recording, no audio).

I was inspired by the val-loader from webpack.

Related Projects

• preval.macro - nicer integration with babel-plugin-macros
• define_macro-loader - Integrate preval with C-Like macros, thus, increasing preval use cases.

Other Solutions

I'm not aware of any, if you are please make a pull request and add it here!

Issues

Looking to contribute? Look for the Good First Issue label.

🐛 Bugs

Please file an issue for bugs, missing documentation, or unexpected behavior.

See Bugs

💡 Feature Requests

Please file an issue to suggest new features. Vote on feature requests by adding a 👍. This helps maintainers prioritize what to work on.

See Feature Requests

Contributors ✨

Thanks goes to these people (emoji key):

Kent C. Dodds 💻 📖 🚇 ⚠️	Matt Phillips 💻 📖 ⚠️	Philip Oliver 🐛	Sorin Davidoi 🐛 💻 ⚠️	Luke Herrington 💡	Lufty Wiranda 💻	Oscar 💻 ⚠️
pro-nasa 📖	Sergey Bekrin	Mauro Bringolf 💻 ⚠️	Joe Lim 💻	Marcin Zielinski 💻	Tommy 💻	Matheus Gonçalves da Silva 📖
Justin Dorfman 🔍	Andrew Rottier 📖	Michaël De Boey 💻	Braydon Hall 💻	Jacob M-G Evans 💻	Juhana Jauhiainen 💻	Peter Hozák 💻
Michael Peyper 💻	Marcelo Silva Nascimento Mancini 📖 🔌	Minh Nguyen 💻 ⚠️ 🚇

This project follows the all-contributors specification. Contributions of any kind welcome!

LICENSE

MIT