TypeError: redeclaration of let error in Firebug console if running ES6 code

Question

I am learning ES6, so bear me please.

Following is the code which is running fine, if I click the Run button one time, but on second hit it starts showing

Answer 1

ES6 does not allow you to do this (redeclaring a block-scoped variable in the same scope):

let foo;
let foo;

And since the console keeps state until you reload the page (you are in the context of the page after all), the first time you run it you define myArr so you cannot redefine it on the second run.