Explanation of `let` and block scoping with for loops

Question

I understand that let prevents duplicate declarations which is nice.

let x;
let x; // error!

Variables declared with let

Answer 1

Is this just syntactic sugar for ES6?

No, it's more than syntactic sugar. The gory details are buried in §13.6.3.9 CreatePerIterationEnvironment.

How is this working?

If you use that let keyword in the for statement, it will check what names it does bind and then

• create a new lexical environment with those names for a) the initialiser expression b) each iteration (previosly to evaluating the increment expression)
• copy the values from all variables with those names from one to the next environment

Your loop statement for (var i = 0; i < 10; i++) process.nextTick(_ => console.log(i)); desugars to a simple

// omitting braces when they don't introduce a block
var i;
i = 0;
if (i < 10)
    process.nextTick(_ => console.log(i))
    i++;
    if (i < 10)
        process.nextTick(_ => console.log(i))
        i++;
        …

while for (let i = 0; i < 10; i++) process.nextTick(_ => console.log(i)); does "desugar" to the much more complicated

// using braces to explicitly denote block scopes,
// using indentation for control flow
{ let i;
  i = 0;
  __status = {i};
}
{ let {i} = __status;
  if (i < 10)
      process.nextTick(_ => console.log(i))
      __status = {i};
}   { let {i} = __status;
      i++;
      if (i < 10)
          process.nextTick(_ => console.log(i))
          __status = {i};
    }   { let {i} = __status;
          i++;
          …