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++;
…
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