javascript - SyntaxError: Use of const in strict mode

Question

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?

Edit:

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

Answer 1

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.