Transform arrow function in variable declaration to named function

[mpodlasin]

This is a work-in-progress for #6433.

I wanted to start with the simplest case of transforming

const x = () => {}

into

var x = function x() {}; // currently it emits "var x = function() {}" (note lack of function name)

I was able to get desired result in baselines, but many tests fail with

Error: Debug Failure. False expression: position cannot precede the beginning of the file

I imagine this is a problem with position of synthesized identifier for function name, but I have no idea how should I fix it. Many transforms create nodes (for example object literal transform creates many assignments) and yet they do not have to manipulate positions of synthesized nodes.

I am super confused. Help! I need some pointers how to proceed. @rbuckton @mhegazy

Thanks in advance!

[mhegazy]

@rbuckton can you take a look.

[mpodlasin]

Ok. It seems I have figured it out. Tests should pass now.
I however still need confirmation that general direction of changes is ok. :)

[kpreisser]

Hi,

I think there needs to be a special treatment for unnamed functions that reference the variable in which they are stored:

var x = recursive => {
    if (!recursive)
        return x(true);
    else
        return 1;
}
var y = x;

x = () => {
    return 2;
};

console.log(y(false));

Expected output (when running the code directly in Node, or running the transpiled ES5 code without this PR in Node):

2

Actual output with this PR, when running the transpiled ES5 code in Node:

1

Maybe in this case the x variable needs to be renamed (like it is done when having multiple let declarations with the same name).

var x_1 = function x(recursive) {
    if (!recursive)
        return x_1(true);
    else
        return 1;
};
var y = x_1;
x_1 = function () {
    return 2;
};
console.log(y(false));

Thanks!

[mhegazy]

I am afraid the fix here is not as simple as just adding a name. As @kpreisser noted, the name could be changed outside the function.. moreover, adding the name may shadow another variable that was visible otherwise.. e.g. var a; var o = { a: () => { a } }.

Looking at this again, i am not sure this is the right solution. The other solution is setting the name using object.defineProperty, but that too, not sure we want to litter the generated code with such calls.. It may be better to just do nothing on this issue in the time being.

[mhegazy]

closing for now, as this has grown stale, and it is an incomplete fix in the first place.