About var/const, why this code will work?

Question

Open dev console (or in node.js), enter the following code by sequence:

a = 1
var a = 2
const a = 3

As you can see it works, but if you enter all the code at once, you will get an error, which I think is more reasonable

I guess the only difference is hoisting, if you input all the code at once, it will be treated as:

var a
a = 1
a = 2
const a = 3

But I don't understand why it works when you enter line by line, also if you skip the first line, you get an error as expected

Answer 1

In global scope of the environment a variable can be assigned and declared irrespective of the type, therefore when you enter following code in sequence, it works.

z = 1
var z = 2
const z = 3

value of z is 3 now

Now lets say we execute all this at once

    z = 4;  var z = 5;  const z = 6;

value of z is still 3

the interpreter in this case will throw error because here it will try to execute all the above statement once as a block and now it has found multiple declaration for 'z' and thus will throw an error for the whole block and will not execute even the first part i.e z = 4;

the statement where everything is executed at once is equivalent to

(function foo(){ z = 4; var z = 5; const z = 6;  })();