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.

Author: Kent C. Dodds

README.md

@@ -94,8 +94,6 @@ See more below.
 - [Use with `babel-plugin-macros`](#use-with-babel-plugin-macros)
 - [Examples](#examples)
 - [Notes](#notes)
-- [Limitations](#limitations)
-  - [Code transpilation](#code-transpilation)
 - [FAQ](#faq)
   - [How is this different from prepack?](#how-is-this-different-from-prepack)
   - [How is this different from webpack loaders?](#how-is-this-different-from-webpack-loaders)
@@ -122,9 +120,8 @@ Important notes:
 
 1.  All code run by `preval` is _not_ run in a sandboxed environment
 2.  All code _must_ run synchronously.
-3.  The code string that preval evaluates will be transpiled, however, only that
-    code string will be prevaled. Learn more about this in the
-    [limitations](#limitations) section below.
+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](https://www.youtube.com/watch?v=1queadQ0048&list=PLV5CVI1eNcJgCrPH_e6d57KRUTiDZgs0u)
@@ -339,77 +336,13 @@ Ok:
 }
 ```
 
-## Limitations
-
-### Code transpilation
-
-It's recommended that code you want to preval be written in a way that works in
-the version of Node that you're running. That said, some code will be
-transpiled. If you for some strange reason _really_ want to write your preval
-code in a way that requires it to be transpiled, then expand here:
-
-<details>
-<summary>Expand (beware)</summary>
-
-Here are a few examples of what will and wont be transpiled by preval:
-
-> This is assuming that you've configured babel to support ESModule syntax but
-> you're using a node version which does not support ESModules natively.
-
-```javascript
-// a.js // this file can use ESModules. Regular babel handles that.
-import value from /* preval */ './b'
-
-// b.js // this file can use ESModules. preval handles that.
-import c from './c'
-export default c
-
-// c.js // this file cannot have ESModules in it.
-// Neither preval nor babel will transpile this file's contents
-module.exports = 'c'
-```
-
-```javascript
-// this file can use ESModules
-var x = preval`
-  // this string can use ESModules
-  import b from 'b.js'
-  // however b.js and it's dependents cannot
-`
-```
-
-```javascript
-// this file can use ESModules
-// and b.js will be transpiled and evaluated
-// by preval so it can use ESModules too
-// but if b.js has dependencies then it cannot.
-var x = preval.require('./b.js')
-```
-
-It's also notable that you can use preval in preval:
-
-```javascript
-// this file can use ESModules
-var x = preval`
-  // this string can use ESModules
-  import b from /* preval */ 'b.js'
-  // b can use ESModules (but not it's dependencies).
-`
-```
-
-Again, this is admittedly pretty confusing, so it's strongly recommended that
-any files you preval are written in a way that they work in node without needing
-transpilation.
-
-</details>
-
 ## FAQ
 
 ### How is this different from prepack?
 
-[`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.
+[`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
@@ -420,14 +353,15 @@ possible in the browser (or with prepack), but `preval` enables you to do this.
 This plugin was inspired by webpack's [val-loader][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`][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`).
+you're probably not bundling your code with [`webpack`][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`][webpack-loaders].
+If you want to learn more, check `webpack` documentations about
+[`loaders`][webpack-loaders].
 
 ## Inspiration
 

src/__tests__/.babelrc

@@ -17,7 +17,7 @@ const x = preval\`
 
 const x = {
   "val": "# fixture\\n\\nThis is some file thing...\\n",
-  "getSplit": function getSplit(splitDelimiter) {
+  "getSplit": function (splitDelimiter) {
     return x.val.split(splitDelimiter);
   }
 };
@@ -34,19 +34,6 @@ const x = "<PROJECT_ROOT>/src/__tests__/index.js";
 
 `;
 
-exports[`preval can use preval in preval: can use preval in preval 1`] = `
-
-var x = preval\`
-  import message from /* preval */ './fixtures/es6'
-  export default message
-\`
-
-      ↓ ↓ ↓ ↓ ↓ ↓
-
-var x = "this string used exported default";
-
-`;
-
 exports[`preval dynamic error: dynamic error 1`] = `
 
 const x = preval\`module.exports = "\${dynamic}"\`
@@ -70,6 +57,16 @@ module.exports = undefined;
 
 `;
 
+exports[`preval handles transpiled modules (uses default export): handles transpiled modules (uses default export) 1`] = `
+
+let one = preval.require('./fixtures/es6-identity.js', 1)
+
+      ↓ ↓ ↓ ↓ ↓ ↓
+
+let one = 1;
+
+`;
+
 exports[`preval import comment (with extras before): import comment (with extras before) 1`] = `
 
 import x from /* this is extra stuff */ /* preval */ "./fixtures/compute-one.js"
@@ -242,32 +239,6 @@ preval\`module.exports = "foo"\`
 
 `;
 
-exports[`preval transpiled: transpiled 1`] = `
-
-export default preval\`
-  import path from 'path'
-  export default path.join(__dirname, 'something/whatever.js').replace(__dirname, '')
-\`
-
-      ↓ ↓ ↓ ↓ ↓ ↓
-
-export default "/something/whatever.js";
-
-`;
-
-exports[`preval transpiles string contents: transpiles string contents 1`] = `
-
-var x = preval\`
-  import one from './fixtures/compute-one'
-  export default one
-\`
-
-      ↓ ↓ ↓ ↓ ↓ ↓
-
-var x = 1;
-
-`;
-
 exports[`preval with flow after: with flow after 1`] = `
 
 // @preval

src/__tests__/__snapshots__/index.js.snap

@@ -1,5 +1,11 @@
-export default acceptsArg
+/* eslint-disable */
+// codegen can't accept non-transpiled esmodules
+// but it can handle transpiled esmodules
+// so we're simulating a pre-transpiled module here
+Object.defineProperty(exports, '__esModule', {
+  value: true,
+})
 
-function acceptsArg(arg) {
-  return arg
+exports.default = function(input) {
+  return input
 }

src/__tests__/fixtures/es6-identity.js

@@ -39,7 +39,6 @@ pluginTester({
       \`
     `,
     'a-bunch-of-checks': fixture('a-bunch-of-checks'),
-    transpiled: fixture('transpiled'),
     'dynamic error': error(
       `const x = preval\`module.exports = "\${dynamic}"\``,
     ),
@@ -93,17 +92,8 @@ pluginTester({
       // @preval
       /* comment */
     `),
-    'transpiles string contents': `
-      var x = preval\`
-        import one from './fixtures/compute-one'
-        export default one
-      \`
-    `,
-    'can use preval in preval': `
-      var x = preval\`
-        import message from /* preval */ './fixtures/es6'
-        export default message
-      \`
+    'handles transpiled modules (uses default export)': `
+      let one = preval.require('./fixtures/es6-identity.js', 1)
     `,
   },
 })

src/__tests__/fixtures/es6.js

@@ -1,25 +1,18 @@
 const p = require('path')
-const requireFromString = require('require-from-string')
+const requireFromStringOfCode = require('require-from-string')
 const objectToAST = require('./object-to-ast')
 
 module.exports = {
   getReplacement,
-  transformAndRequire,
+  requireFromString,
 }
 
-function transformAndRequire({
+function requireFromString({
   string: stringToPreval,
-  fileOpts,
+  fileOpts: {filename},
   args = [],
-  babel,
 }) {
-  const {filename, plugins, presets} = fileOpts
-  const {code} = babel.transform(stringToPreval, {
-    filename,
-    plugins,
-    presets,
-  })
-  let mod = requireFromString(code, filename)
+  let mod = requireFromStringOfCode(String(stringToPreval), filename)
   mod = mod && mod.__esModule ? mod.default : mod
 
   if (typeof mod === 'function') {
@@ -39,6 +32,6 @@ function transformAndRequire({
 }
 
 function getReplacement({string, fileOpts, args, babel}) {
-  const mod = transformAndRequire({string, fileOpts, args, babel})
+  const mod = requireFromString({string, fileOpts, args, babel})
   return objectToAST(mod, {babel, fileOptions: fileOpts})
 }

src/__tests__/fixtures/transpiled.js

@@ -1,7 +1,7 @@
 const p = require('path')
 const fs = require('fs')
 // const printAST = require('ast-pretty-print')
-const {getReplacement, transformAndRequire} = require('./helpers')
+const {getReplacement, requireFromString} = require('./helpers')
 
 module.exports = prevalPlugin
 
@@ -88,10 +88,9 @@ function prevalPlugin(babel) {
         let argValues
         if (prevalComment !== 'preval') {
           const arg = prevalComment.replace(/preval\((.*)\)/, '$1').trim()
-          const argValue = transformAndRequire({
+          const argValue = requireFromString({
             string: `module.exports = ${arg}`,
             fileOpts,
-            babel,
           })
           argValues = [argValue]
         }