JAVA SCRIPT - Using let and Other ES 6 Additions in Node

 Using let and Other ES 6 Additions in Node 

Problem

You want to use some of the new ECMAScript 6 functionality, such as let in your Node application, but they don’t seem to work.

Solution

You’ll need to use two command-line options when you run the Node application: harmony, to add in support for whatever ECMAScript Harmony features are currently implemented, and use-strict to enforce strict JavaScript processing:

node --harmony --use-strict open.js

Or you can trigger strict mode by adding the following line as the first in the application:

'use strict';

EXPLAIN

Internally, Node runs on V8, Google’s open source JavaScript engine. You might assume that the engine implements most if not all of the newest cutting-edge JavaScript func‐ tionality, including support for let. 

Once the --harmony option has been given, you can use let instead of var. However, you must also use strict mode to use let, either by providing the command-line flag, or using use strict in the application:And it is true that Google has implemented much of the newest JavaScript functionality. 

However, some of the newer functionality isn’t available to a Node application unless you specify the harmony command-line option, similar to having to turn the option on in your browser. You can find this and other options by typing the following at the command line:

'use strict';
let fs = require('fs');
fs.readFile('main.txt', {encoding: 'utf8', flag: 'r+'},function(err,data) {
 if (err) {
 console.log(err.message);
 } else {
 console.log(data);
 }
});