const / let scopes are different from native JS engines (V8 and SpiderMonkey)

[gfx]

TypeScript Version: 3.0.1

Search Terms: variable scope es2015 label:Bug

Code

function f() {
  console.log("foo:", foo);
}

f(); // => foo: undefined
let foo = 42;
f(); // => foo: 42

Expected behavior:

In V8 (nodejs, chrome) and SpiderMonkey (firefox), it can't compile it runs with ReferenceError if foo is not initialized.

Actual behavior:

It compiles and runs, showing foo: undefined and foo: 42.

Playground Link: playground

Related Issues: #19503 looks similar, I guess.

[ghost]

Seems like a duplicate of #11498.
Currently TypeScript never considers when a function is called when checking it. So it assumes any variables it closes over will have been defined by the time the function is called.

If this was instead a suggestion to copy the runtime behavior and throw ReferenceError instead of getting undefined when transpiling to es5, that's probably too expensive to do for every variable that happens to be used in a closure -- and we currently have no way of detecting that a function is called only when its closed-over variables have already been defined, so we would have to do that for every variable.

Note that if you don't need to support old browsers you can set --target es6 which would solve the issue for you.

[typescript-bot]

This issue has been marked as a 'Duplicate' and has seen no recent activity. It has been automatically closed for house-keeping purposes.