ES2015 Const Declarations in Babel

Babel 6

This is probably the version of Babel you are using by the time I'm writing this note.

Take the following piece of code:

        
const a = true
a = false
        
      

You can try to compile it through Babel 6, here is the result:

        
repl: "a" is read-only
  1 | const a = true
> 2 | a = false
    | ^
        
      

It doesn't compile. The result is understandable, we tried to change the value of a and you can't re-assign a constant.

Babel 7

I used the same piece of code, we will see the result if you compile it through Babel 7.

        
// Runtime helpers

function _readOnlyError(name) {
    throw new Error("\"" + name + "\" is read-only");
}

// Code

var a = true;
a = (_readOnlyError("a"), false);
        
      

And as you would expect here is the result when you actually run this code in your browser:

        
Uncaught Error: "a" is read-only
  at _readOnlyError (:4:11)
  at :10:6
        
      

Why did we change it?

Basically the error at compile-time in Babel 6 is now a runtime error in Babel 7.

Take the following code:

        
const a = true

if (false) {
  a = false
}
        
      

This code is perfectly fine and according to the specification, it should run without any errors, because we won't ever reach the constant re-assigning.
Babel 6 wasn't spec compliant in that regard because it will refuse to compile the code and throw an error.

Babel 7 will generate the following code:

        
// Code

var a = true;

if (false) {
    a = (_readOnlyError("a"), false);
}
        
      

You can avoid the runtime error

Of course you shouldn't ever re-assign a constant, but it can happen by mistake.
You can use a linter because most of them will warn you about that.

Credits

This work has been done by maurobringolf, you can see his PR here: babel/babel#5930.