Should check for existence of binding before creating a new one

[jasonwilliams]

Take this javascript..

let a = "foo";
a = "bar";

The error message we get is

thread 'main' panicked at 'Binding already exists!', src/lib/environment/global_environment_record.rs:108:13
note: Run with `RUST_BACKTRACE=1` environment variable to display a backtrace.

This is because rather than re-using a the interpreter is attempting to create a new binding called a which already exists so causes an error.

https://github.com/jasonwilliams/boa/blob/master/src/lib/exec.rs#L333-L337 needs to be more clever. there needs to be a check here to see if the binding already exists, if it does use that binding and set it to a new value.

What does the spec say?

https://tc39.github.io/ecma262/#sec-assignment-operators-runtime-semantics-evaluation
https://tc39.github.io/ecma262/#sec-putvalue

[perfectmak]

@jasonwilliams is this still available, I would like to work on it to better understand how boa works

[jasonwilliams]

Sure go for it @perfectmak