babel-plugin-preval

    5.0.0 • Public • Published

    babel-plugin-preval

    Pre-evaluate code at build-time


    Build Status Code Coverage version downloads MIT License

    All Contributors

    PRs Welcome Code of Conduct Babel Macro Examples

    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

    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

    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

    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

    💻

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

    LICENSE

    MIT

    Install

    npm i babel-plugin-preval

    DownloadsWeekly Downloads

    39,616

    Version

    5.0.0

    License

    MIT

    Unpacked Size

    31.9 kB

    Total Files

    9

    Last publish

    Collaborators

    • kentcdodds