ES6 Modules: Undefined onclick function after import
JavascriptEcmascript 6Es6 ModulesJavascript Problem Overview
I am testing ES6 Modules and want to let the user access some imported functions using onclick:
test.html:
<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Module Test</title>
</head>
<body>
<input type="button" value="click me" onclick="hello();"/>
<script type="module">import {hello} from "./test.js";</script>
</body>
</html>
test.js:
export function hello() {console.log("hello");}
When I click the button, the developer console says: ReferenceError: hello is not defined. How can I import functions from modules so that they are available as onclick functions?
I am using Firefox 54.0 with dom.moduleScripts.enabled set to true.
Javascript Solutions
Solution 1 - Javascript
Module creates a scope to avoid name collisions.
Either expose your function to window object
import {hello} from './test.js'
window.hello = hello
Or use addEventListener to bind handler. Demo
<button type="button" id="hello">Click Me</button>
<script type="module">
import {hello} from './test.js'
document.querySelector('#hello').addEventListener('click', hello)
</script>
Solution 2 - Javascript
Module scope in ES6 modules:
When you import a script in the following manner using type="module":
<script type="module">import {hello} from "./test.js";</script>
You are creating a certain scope called module scope. Here is where modules scope is relative to other levels of scope. Starting from global they are:
- Global scope: All declarations outside a module on the outermost level (i.e. not in a function, and for
constandletnot in a block) are accessible from everywhere in the Javascript runtime environment - Module scope: All declarations inside a module are scoped to the module. When other javascipt tries to access these declarations it will throw a reference error.
- Function scope: All the variables declared inside (the top level of) a function body are function scoped
- Block scope:
letandconstvariables are block scoped
You were getting the referenceError because the hello() function was declared in the module, which was module scoped. As we saw earlier declarations inside module scope are only available within that module, and you tried to use it ouside the module.
We can make declarations inside a module global when we explicitly put it on the window object so we can use it outside of the module. For example:
window.hello = hello; // putting the hello function as a property on the window object
Solution 3 - Javascript
While the accepted answer is correct, it scales poorly once you start importing from multiple modules, or declaring multiple functions . Plus, as noted by @Quentin, it risks global name space pollution.
I prefer a slight modification
import { doThis, doThat } from './doStuff.js'
import { foo1, foo2 } from './foo.js'
function yetAnotherFunction() { ... }
window._allTheFns = { doThis, doThat, foo1, foo2, yetAnotherFunction }
// or, if you prefer, can subdivide
window._doStuffjs = { doThis, doThat }
window._foojs = { foo1, foo2 }
- Uses an "unusual" property name to (hopefully) avoid global namespace issues
- No need to immediately attach (or forget to attach) an EventListener.
- You can even put the listener in the HTML, e.g.
<button onclick="window._foojs.foo1(this)">Click Me</button>