443726 - TypeError: redeclaration of const FOO should occur at runtime if redeclaration really happens

Status: RESOLVED INVALID

(Core :: JavaScript Engine, defect)

Description

[arno renevier]

Hi,
when trying to execute that script:

var bool = true;
if (bool) {
    const FOO = "bar";
} else {
    const FOO = "baz";
}

javascript engine throws before running the script.
file.js:5: TypeError: redeclaration of const FOO:
file.js:5:     const FOO = "baz";
file.js:5: ..........^

but redeclaration would never occur at runtime.

That makes it impossible to have libraries defining constant as following:
                                                                                                                                            
if (typeof (lib_alreadyloaded) == "undefined") {
    const lib_alreadyloaded = true;
    const FOO = "bar";
}

if library is loaded a second time, script execution will break. When working whith multiple overlays, it's sometimes hard to keep track of which script have already been included, and making it possible to include such scripts would help in that case.

Comment 1

[Brendan Eich [:brendan]]

For ES4 (JS2) the proposal is that const is block-scoped, cannot be used before set, but need not be set in its declaration (is a write-once variable). This means your example is useless, since FOO (neither declaration) escapes its block scope in the then or else clause of "if (bool)".

We will be changing SpiderMonkey to track ES4 in bug 229756. Making this bug depend on that one for now.

/be

Comment 2

[Tom S. (please needinfo tschuster)]

const is now block scoped, so this doesn't throw.