Es2015 Import Doesn't Work (Even At Top-Level) in Firefox

ES2015 import doesn't work (even at top-level) in Firefox

Actually the error you got was because you need to explicitly state that you're loading a module - only then the use of modules is allowed:

<script src="t1.js" type="module"></script>

I found it in this document about using ES6 import in browser. Recommended reading.

Fully supported in those browser versions (and later; full list on caniuse.com):

Firefox 60
Chrome (desktop) 65
Chrome (android) 66
Safari 1.1

In older browsers you might need to enable some flags in browsers:

Chrome Canary 60 – behind the Experimental Web Platform flag in chrome:flags.
Firefox 54 – dom.moduleScripts.enabled setting in about:config.
Edge 15 – behind the Experimental JavaScript Features setting in about:flags.

How can I fix 'import declarations may only be on top level` ECMAScript import errors

To make it work all you need to do is to mark both of these JS files as module in your index.html file and it will work fine.

 <html>
<body>
    <script>
        function f() {
            alert("IT WORKS")
        }
    </script>

    <script src="f.js" type="module"></script>
    <script src="j.js" type="module"></script>
  </body>

ES2015 import not working in node v6.0.0 with with --harmony_modules option

They're just not implemented yet.

Node 6.0.0 uses a version of V8 with most of ES6 features completed. Unfortunately modules isn't one of those completed features.

node --v8-options | grep harmony

in progress harmony flags are not fully implemented and usually are not working:

--es_staging (enable test-worthy harmony features (for internal use only))

--harmony (enable all completed harmony features)

--harmony_shipping (enable all shipped harmony features)

--harmony_object_observe (enable "harmony Object.observe" (in progress))

--harmony_modules (enable "harmony modules" (in progress))

--harmony_function_sent (enable "harmony function.sent" (in progress))

--harmony_sharedarraybuffer (enable "harmony sharedarraybuffer" (in progress))

--harmony_simd (enable "harmony simd" (in progress))

--harmony_do_expressions (enable "harmony do-expressions" (in progress))

--harmony_iterator_close (enable "harmony iterator finalization" (in progress))

--harmony_tailcalls (enable "harmony tail calls" (in progress))

--harmony_object_values_entries (enable "harmony Object.values / Object.entries" (in progress))

--harmony_object_own_property_descriptors (enable "harmony Object.getOwnPropertyDescriptors()" (in progress))

--harmony_regexp_property (enable "harmony unicode regexp property classes" (in progress))

--harmony_function_name (enable "harmony Function name inference")

--harmony_regexp_lookbehind (enable "harmony regexp lookbehind")

--harmony_species (enable "harmony Symbol.species")

--harmony_instanceof (enable "harmony instanceof support")

--harmony_default_parameters (enable "harmony default parameters")

--harmony_destructuring_assignment (enable "harmony destructuring assignment")

--harmony_destructuring_bind (enable "harmony destructuring bind")

--harmony_tostring (enable "harmony toString")

--harmony_regexps (enable "harmony regular expression extensions")

--harmony_unicode_regexps (enable "harmony unicode regexps")

--harmony_sloppy (enable "harmony features in sloppy mode")

--harmony_sloppy_let (enable "harmony let in sloppy mode")

--harmony_sloppy_function (enable "harmony sloppy function block scoping")

--harmony_proxies (enable "harmony proxies")

--harmony_reflect (enable "harmony Reflect API")

--harmony_regexp_subclass (enable "harmony regexp subclassing")

Es2015 Import Doesn't Work (Even At Top-Level) in Firefox