While support for ES6 modules arrived as an experimental feature in Node.js 8.5, there are two ways to use these modules on earlier Node.js implementations. 

One method is to use the Babel transpiler to rewrite ES6 code so it can execute on older Node.js versions. For an example, see https://blog.revillweb.com/using-es2015-es6-modules-with-babel-6-3ffc0870095b.

The better method is the esm package in the Node.js registry. Simply do the following:

$ nvm install 6
Downloading and installing node v6.14.1...
Downloading https://nodejs.org/dist/v6.14.1/node-v6.14.1-darwin-x64.tar.xz...
######################################################################## 100.0%
Computing checksum with shasum -a 256
Checksums matched!
Now using node v6.14.1 (npm v3.10.10)
$ nvm use 6
Now using node v6.14.1 (npm v3.10.10)
$ npm install esm
... npm output
$ node --require esm simpledemo.mjs 
Hello, world!
1 1
2 4
2 4
3 9
4 16
5 25
42

To use this module, one simply invokes require('esm') once, and ES6 modules are retrofitted into Node.js.  The --require flag automatically loads the named module.  Without rewriting the code, we can selectively use the esm module with this the command-line option.  

This example demonstrates retrofitting ES6 modules into older Node.js releases.  To successfully execute the ls.mjs example   we must have support for async/await functions, and arrow functions.  Since Node.js 6.x does not support either, the ls.mjs example will fail, and will necessitate rewriting such code:

$ node --version
v6.14.1
$ node -r esm ls.mjs 
/Users/David/chap03/ls.mjs:5
(async () => {
       ^

SyntaxError: Unexpected token (
 at exports.runInThisContext (vm.js:53:16)
 at Module._compile (module.js:373:25)

For more information, see: 
https://medium.com/web-on-the-edge/es-modules-in-node-today-32cff914e4b.  That article describes an older release of the esm module, at the time named @std/esm.