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.