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 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.
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:
- All code run by
preval
is not run in a sandboxed environment - All code must run synchronously.
- All code will be transpiled via
babel-core
directly orbabel-register
and should follow all of the normal rules for.babelrc
resolution (the closest.babelrc
to the file being run is the one that's used). This means you can rely on any babel plugins/transforms that you're used to using elsewhere in your codebase.
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" };
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']
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'
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
.babelrc
{
"plugins": ["preval"]
}
babel --plugins preval script.js
require('babel-core').transform('code', {
plugins: ['preval'],
})
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.
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 twitch.
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 💻 📖 🚇 |
Matt Phillips 💻 📖 |
Philip Oliver 🐛 |
---|
This project follows the all-contributors specification. Contributions of any kind welcome!
MIT