Attempting to override '{a}' which is a constant

When do I get this error?

The "Attempting to override '{a}' which is a constant" error is thrown when JSHint encounters an assignment expression with an identifer that has been declared in a constant variable statement. In the following example we declare a constant MY_CONST and assign a value to it, and then attempt to change its value:

/*jshint esnext: true */
const MY_CONST = 10;
MY_CONST = 20;

Why do I get this error?

This error is raised to highlight a fatal JavaScript type error. Your code will fail to run if you do not resolve this error. Mozilla Developer Network offers the following note:

The value of a constant cannot change through re-assignment, and a constant cannot be re-declared.

You can fix this issue by removing any assignments to constants declared with the const keyword:

/*jshint esnext: true */
const MY_CONST = 10;

However, since browser support for the const statement is limited and most implementations currently differ significantly from the upcoming ECMAScript 6 specification, it's recommended that you don't use it all, and simply use the var statement instead. A common convention to indicate a variable with a value that shouldn't change is to give that variable an identifier made up of uppercase characters, as has been done in the previous examples:

/*jshint esnext: true */
var MY_CONST = 10; // A fake constant

In JSHint 1.0.0 and above you have the ability to ignore any warning with a special option syntax. Since this message relates to a fatal syntax error you cannot disable it.


About the author

James Allardice

This article was written by James Allardice, JavaScript engineer at Mammal in London. Passionate about AngularJS, Node and writing clean and maintainable JavaScript and uses JSHint every day to help achieve this.