I'm trying to use import and export to create modules and it's not working.
I added .24.0/babel.min.js to the index.html header and tried to import a js file and get an error message saying SyntaxError: import declarations may only appear at top level of a module. What can I possibly be doing wrong?
I know I can use require.js but rather use import and export.
HTML
script src=".24.0/babel.min.js"></script
JS File
import Mymodule from './modules/mymodule';
I'm trying to use import and export to create modules and it's not working.
I added https://cdnjs.cloudflare./ajax/libs/babel-standalone/6.24.0/babel.min.js to the index.html header and tried to import a js file and get an error message saying SyntaxError: import declarations may only appear at top level of a module. What can I possibly be doing wrong?
I know I can use require.js but rather use import and export.
HTML
script src="https://cdnjs.cloudflare./ajax/libs/babel-standalone/6.24.0/babel.min.js"></script
JS File
import Mymodule from './modules/mymodule';
- The error means "import" should appear in the top level of a file not in a Class or a Function. – 喝茶的螃蟹 Commented May 9, 2017 at 1:07
- I have my import statement in a js file with no other code. Just this import Mymodule from './modules/mymodule'; – icode Commented May 9, 2017 at 1:13
2 Answers
Reset to default 7The scripts that babel-standalone translates execute by default in global scope, so any symbols defined by them are automatically available to every other module. From that perspective, you don't need import/export statements in your modules.
However, you might be trying to maintain source files that can be used both by babel-standalone (e.g. for quick test environments, feature demonstrations, etc) and via bundlers such as webpack. In that case, you need to keep the import and export statements there for patibility.
One way to make it work is to add extra symbols into the global scope that cause the import and export code that babel generates to have no effect (rather than causing an error as usually occurs). For example, export statements are piled into code that looks like this:
Object.defineProperty (exports, "__esModule", {
value: true
});
exports.default = MyDefaultExportedClass;
This fails if there is no existing object called "exports". So give it one: I just give it a copy of the window object so anything interesting that gets defined is still accessible:
<script>
// this must run before any babel-piled modules, so should probably
// be the first script in your page
window.exports = window;
import statements are translated to calls to require(). The result (or properties extracted from it) is assigned to the variable used as the identifier in the import statement. There's a little bit of plication around default imports, which are different depending on whether or not the result of require() contains the property __esModule. If it doesn't, things are easier (but then you can't support having both default and named exports in the same module ... if you need to do this, look at the code babel produces and figure out how to make it work).
So, we need a working version of require(). We can provide one by giving a static translation of module name to exported symbol/symbols. For example, in a demo page for a React ponent, I have the following implementation:
function require (module) {
if (module === "react") return React;
if (module === "react-dom") return ReactDOM;
}
For a module returning multiple symbols, you'd just return an object containing the symbols as properties.
This way, a statement like
`import React from "react";`
translates to code that is effectively:
`React = React;`
which is roughly what we want.
Babel cannot perform client-side transpiling of modules, or rather it is not universally supported by browsers. In fact, unless you use a plugin, Babel will transform import into require().
If I run the following code:
<head>
<script src="https://cdnjs.cloudflare./ajax/libs/babel-standalone/6.24.0/babel.js"></script>
<script defer type="text/babel" data-presets="es2015">
import Mymod from './modules/module';
Mymod();
</script>
</head>
I get the following error:
Uncaught ReferenceError: require is not defined
From Babel Docs:
Compiling in the browser has a fairly limited use case, so if you are working on a production site you should be prepiling your scripts server-side. See setup build systems for more information.
Most people choose a pre-piled module bundler like Webpack or Rollup.
If you really want to perform this client-side, use RequireJS with Babel run via a plugin, though you may need to use AMD syntax.
Native browser support for ES6 modules is still in early stages. But to my knowledge there isn't a preset/plugin available yet for Babel to tell it not to transform import/export statements.