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

Question

I am using node v6.0.0 and wanted to use ES2016 (ES6). However I realized that the "import" syntax is not working. Isn't "import" fundamental to for writing modular code in ES2015? I tried running node with --harmony_modules option as well but still got the same error about "import". Here's the code.

Working code without "import":

'use strict';
let sum = 0;
class Number {

  addNumber(num1, num2) {
    return num1 + num2;
  }
}
let numberObj = new Number();
sum = numberObj.addNumber(1,2);
console.log("sum of two number 1 and 2 "+ sum);

Notworking code with "import":

server.js

'use strict';
import Number from "./Number";

let sum = 0;


let numberObj = new Number();

sum = numberObj.addNumber(1,2);
console.log("sum of two number 1 and 2 "+ sum);

Number.js

'use strict';
export default class Number {

  addNumber(num1, num2) {
    return num1 + num2;
  }
}

I also checked http://node.green/ to see the supported es6 however not able to understand why it doesn't work with --harmony_modules option. Please help.

Answer 1

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")

Answer 2

This should be a comment to @Paulpro's answer but I do not have enough rep to post a comment.

For Windows users the equivalent command is:

node --v8-options | findstr harmony

Answer 3

Until modules are implemented you can use the Babel "transpiler" to run your code:

npm install --save babel-cli babel-preset-node6
./node_modules/.bin/babel-node --presets node6 ./your_script.js

See https://www.npmjs.com/package/babel-preset-node6 and https://babeljs.io/docs/usage/cli/

Downsides: this has various downsides, such as extra compilation time, which can be significant and you now need source maps for debugging; just saying.

Answer 4

As is stated above, ES6 modules are not implemented yet.

It appears to be a non-trivial issue to implement ES6 modules in a way that would be backward-compatible with Common JS modules, which is the current Node.js module syntax.

However, there is a draft of an implementation, that introduces a new file extension - .mjs - for a files containing ES6 modules.

Also, there is a counter-proposal that present an alternative approach of declaring all files with ES6 modules in package.json like so:

{
    "modules.root": "/path/to/es6/modules"
}