SyntaxError: Use of const in strict mode

I'm working with node.js, and in one of my js files I'm using const in "strict mode". When trying to run it, I'm getting an error:

SyntaxError: Use of const in strict mode.

What is the best practice to do this?


'use strict'
const MAX_IMAGE_SIZE = 1024*1024; // 1 MB

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,

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 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.

I had a similar issue recently and ended up in this Q&A. This is not the solution the OP was looking for but may help others with a similar issue.

I'm using PM2 to run a project and in a given staging server I had a really old version of Node, NPM and PM2. I updated everything, however, I kept keeping the same error:

SyntaxError: Use of const in strict mode.

I tried to stop and start the application several times. Also tried to update everything again. Nothing worked. Until I noticed a warning when I ran pm2 start:

>>>> In-memory PM2 is out-of-date, do:
>>>> $ pm2 update
In memory PM2 version: 0.15.10
Local PM2 version: 3.2.9

Gotcha! After running pm2 update, I finally was able to get the application running as expected. No "const in strict mode" errors anymore.

const is not supported by ECMAScript. So after you specify strict mode, you get syntax error. You need to use var instead of const if you want your code to be compatible with all browsers. I know, not the ideal solution, but it is what it is. There are ways to create read-only properties in JavaScript (see Can Read-Only Properties be Implemented in Pure JavaScript?) but I think it might be overkill depending on your scenario.

Below is browser compatibility note from MDN:

Browser compatibility

The current implementation of const is a Mozilla-specific extension and is not part of ECMAScript 5. It is supported in Firefox & Chrome (V8). As of Safari 5.1.7 and Opera 12.00, if you define a variable with const in these browsers, you can still change its value later. It is not supported in Internet Explorer 6-10, but is included in Internet Explorer 11. The const keyword currently declares the constant in the function scope (like variables declared with var).

Firefox, at least since version 13, throws a TypeError if you redeclare a constant. None of the major browsers produce any notices or errors if you assign another value to a constant. The return value of such an operation is that of the new value assigned, but the reassignment is unsuccessful only in Firefox and Chrome (at least since version 20).

const is going to be defined by ECMAScript 6, but with different semantics. Similar to variables declared with the let statement, constants declared with const will be block-scoped.

The use of const in strict mode is available with the release of Chrome 41. Currently, Chrome 41 Beta is already released and supports it.

cd /
npm install -g nave
nave use 6.11.1
node app.js