Why do some variables declared using let inside a function become available in another function, while others result in a reference error?

Question

I can\'t understand why variables act so strange when declared inside a function.

1. In the first function I declare with let the variabl

Answer 1

In the function first(), variables band c are created on the fly, without using var or let.

let a = b = c = 10; // b and c are created on the fly

Is different than

let a = 10, b = 10, c = 10; // b and c are created using let (note the ,)

They become implicit global. That's why they are available in second()

From documentation

Assigning a value to an undeclared variable implicitly creates it as a global variable (it becomes a property of the global object) when the assignment is executed.

To avoid this, you can use "use strict" that will provide errors when one use an undeclared variable

"use strict"; // <-------------- check this

function first() {
   /*
    * With "use strict" c is not defined.
    * (Neither is b, but since the line will be executed from right to left,
    * the variable c will cause the error and the script will stop)
    * Without, b and c become globals, and then are accessible in other functions
    */
   let a = b = c = 10;
   var d = 20;
   second();
}

function second() {
   console.log(b + ", " + c); //reference error
   console.log(a); //reference error
   console.log(d); //reference error
}

first();