Hello es-discuss,
I'm having difficulty figuring out where the ES6 draft spec specifies that a
"use before declaration" error should be thrown. My last understanding of the
temporal dead zone was that ECMAScript would always issue a "use before
declaration" error at runtime, regardless whether it can be statically
determined or not. However it seems like the evaluation semantics of var
declarations, for example, do not lead to any line that throws a ReferenceError.
That is, consider this code:
function f() {
{
var x = 5;
let x;
}
}
f();
I think the var declaration creates a binding for x in the function's lexical
environment, but then binds to the x in the block's environment for the
initialization. As such, the initialization should throw a "use before
declaration" error. But this is what I cannot find in the spec. Maybe I am
wrong about the semantics here?
If I am not wrong then this raises the question of order of operations. Is the
RHS evaluated first, then the error is thrown? Or is the LHS name evaluated
first, throwing if it is a binding that is currently in its TDZ?
E.g. is g() called here before throwing?
function g() {
console.log('hello');
return 0;
}
function f() {
{
var x = g();
let x;
}
}
f();
How about for-in/of loops? Is g() called here?
function g() {
console.log('hello');
return { a: 1, b: 2, c: 3 };
}
function f() {
{
for (var x in g()) {
console.log('unreachable');
}
let x;
}
}
Ian Halliday
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