SyntaxError: Use of const in strict mode

Question

The const and let are part of ECMAScript 2015 (a.k.a. ES6 and Harmony), and was not enabled by default in Node.js 0.10 or 0.12. Since Node.js 4.x, “All shipping [ES2015] features, which V8 considers stable, are turned on by default on Node.js and do NOT require any kind of runtime flag.”. Node.js docs has an overview of what ES2015 features are enabled by default, and which who require a runtime flag. So by upgrading to Node.js 4.x or newer the error should disappear.

To enable some of the ECMAScript 2015 features (including const and let) in Node.js 0.10 and 0.12; start your node program with a harmony flag, otherwise you will get a syntax error. For example:

node --harmony app.js

It all depends on which side your strict js is located. I would recommend using strict mode with const declarations on your server side and start the server with the harmony flag. For the client side, you should use Babel or similar tool to convert ES2015 to ES5, since not all client browsers support the const declarations.

If this is happening in nodejs, it is due to the older version of nodejs. Update node by using,

1) Clear NPM's cache:

sudo npm cache clean -f

2) Install a little helper called 'n'

sudo npm install -g n

3) Install latest stable NodeJS version

sudo n stable

Update nodejs instructions taken from, https://stackoverflow.com/a/19584407/698072

Usually this error occurs when the version of node against which the code is being executed is older than expected. (i.e. 0.12 or older).

if you are using nvm than please ensure that you have the right version of node being used. You can check the compatibility on node.green for const under strict mode

I found a similar issue on another post and posted my answer there in detail

One important step after you update your node is to link your node binary to the latest installed node version

sudo ln -sf /usr/local/n/versions/node/6.0.0/bin/node /usr/bin/node  

This is probably not the solution for everyone, but it was for me.

If you are using NVM, you might not have enabled the right version of node for the code you are running. After you reboot, your default version of node changes back to the system default.

Was running into this when working with react-native which had been working fine. Just use nvm to use the right version of node to solve this problem.

Since the time the question was asked, the draft for the const keyword is already a living standard as part of ECMAScript 2015. Also the current version of Node.js supports const declarations without the --harmony flag.

With the above said you can now run node app.js, with app.js:

'use strict';
const MB = 1024 * 1024;
...

getting both the syntax sugar and the benefits of strict mode.