🐣 Pre-evaluate code at build-time
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kentcdodds feat: drop support for transpiling the code that is being prevaled (#59)
It cause way too many weird bugs that take a TON of time to track down.

BREAKING CHANGE: Code that's being prevaled will no longer be transpiled. You should either transpile it ahead of time or only write the code in a version of JavaScript that runs natively in the version of node you're running.
Latest commit b0f94b9 Jun 27, 2018



Pre-evaluate code at build-time

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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 = {
    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


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


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


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:


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


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!


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']



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


const fileLastModifiedDate = '2017-07-05'

And you can provide some simple dynamic arguments as well:


const fileLastModifiedDate = preval.require(


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.


// @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(


module.exports = 4

Configure with Babel

Via .babelrc (Recommended)


  "plugins": ["preval"]


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 πŸ˜€



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


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


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


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.


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

Other Solutions

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


Thanks goes to these people (emoji key):

Kent C. Dodds

πŸ’» πŸ“– πŸš‡ ⚠️

Matt Phillips

πŸ’» πŸ“– ⚠️

Philip Oliver


Sorin Davidoi

πŸ› πŸ’» ⚠️

Luke Herrington


Lufty Wiranda



πŸ’» ⚠️



Sergey Bekrin

Mauro Bringolf

πŸ’» ⚠️

Joe Lim


Marcin Zielinski




Matheus Gonçalves da Silva


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