babel-plugin-preval
Pre-evaluate code at build-time
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!Babel Macromacros-badgebabel-plugin-macros
!Examplesexamples-badgeexamples
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 youmodule.exports in there will be swapped. For example:const x = preval`module.exports = 1`
// ↓ ↓ ↓ ↓ ↓ ↓
const x = 1Or, 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
- Template Tag - import comment - preval.require - preval file comment (
// @preval)
- Via .babelrc (Recommended)
- Via CLI
- Via Node API
- How is this different from prepack?
- How is this different from webpack loaders?
- 🐛 Bugs
- 💡 Feature RequestsInstallation
This module is distributed via npmnpm which is bundled with nodenode and should be installed as one of your project'sdevDependencies:npm install --save-dev babel-plugin-prevalUsage
Important notes:- All code run by
prevalis not run in a sandboxed environment - All code must run synchronously.
- 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 = 4Exporting a function
If you export a function from a module that you're prevaling (whether usingpreval.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 = () => fnUsage 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.jsVia 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.macropreval.macro if you'd prefer to type less
😀Examples
- Mastodon saved 40kb
babel-plugin-preval
uses preval.macropreval.macro to determine Algolia options based on
process.env.LOCALE. It also uses preval.macropreval.macro to load an
svg file as a string, base64 encode it, and use it as a background-url
for an input element.
build serverless functions using webpack and Babel for development and
production with preval to replace (possible sensible) content in code.Notes
If you usebabel-plugin-transform-decorators-legacy, there is a conflict
because both plugins must be placed at the topWrong:
{
"plugins": ["preval", "transform-decorators-legacy"]
}Ok:
{
"plugins": ["preval", ["transform-decorators-legacy"]]
}FAQ
How is this different from prepack?
prepackprepack 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-loaderval-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 withwebpackwebpack, 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
loaderswebpack-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-loaderval-loader from webpack.
Related Projects
preval.macropreval.macro - nicer integration withbabel-plugin-macrosdefine_macro-loader-
Other Solutions
I'm not aware of any, if you are please make a pull requestprs and add it here!Issues
Looking to contribute? Look for the Good First Issuegood-first-issue label.🐛 Bugs
Please file an issue for bugs, missing documentation, or unexpected behavior.See Bugsbugs
💡 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 Requestsrequests
Contributors ✨
Thanks goes to these people (emoji keyemojis):This project follows the all-contributorsall-contributors specification. Contributions of any kind welcome!
