Default parameters on async functions do not work in Safari

No source file or test is named. Start by reproducing the generated functions in Safari and comparing the async and regular-function output, then locate the compiler path that emits default-parameter handling. Done means async functions with default parameters produce the expected results in Safari without redeclaring an existing parameter.

Consider these two functions.

def multiply(a, b=1):
    return a * b


async def multiply_async(a, b=1):
    return a * b

Compiled to JS, they are:

// Transcrypt'ed from Python, 2024-09-01 01:05:45
import {AssertionError, AttributeError, BaseException, DeprecationWarning, Exception, IndexError, IterableError, KeyError, NotImplementedError, RuntimeWarning, StopIteration, UserWarning, ValueError, Warning, __JsIterator__, __PyIterator__, __Terminal__, __add__, __and__, __call__, __class__, __envir__, __eq__, __floordiv__, __ge__, __get__, __getcm__, __getitem__, __getslice__, __getsm__, __gt__, __i__, __iadd__, __iand__, __idiv__, __ijsmod__, __ilshift__, __imatmul__, __imod__, __imul__, __in__, __init__, __ior__, __ipow__, __irshift__, __isub__, __ixor__, __jsUsePyNext__, __jsmod__, __k__, __kwargtrans__, __le__, __lshift__, __lt__, __matmul__, __mergefields__, __mergekwargtrans__, __mod__, __mul__, __ne__, __neg__, __nest__, __or__, __pow__, __pragma__, __pyUseJsNext__, __rshift__, __setitem__, __setproperty__, __setslice__, __sort__, __specialattrib__, __sub__, __super__, __t__, __terminal__, __truediv__, __withblock__, __xor__, _sort, abs, all, any, assert, bin, bool, bytearray, bytes, callable, chr, delattr, dict, dir, divmod, enumerate, filter, float, getattr, hasattr, hex, input, int, isinstance, issubclass, len, list, map, max, min, object, oct, ord, pow, print, property, py_TypeError, py_iter, py_metatype, py_next, py_reversed, py_typeof, range, repr, round, set, setattr, sorted, str, sum, tuple, zip} from './org.transcrypt.__runtime__.js';
var __name__ = '__main__';
export var multiply = function (a, b) {
	if (typeof b == 'undefined' || (b != null && b.hasOwnProperty ("__kwargtrans__"))) {;
		var b = 1;
	};
	return a * b;
};
export var multiply_async = async function (a, b) {
	if (typeof b == 'undefined' || (b != null && b.hasOwnProperty ("__kwargtrans__"))) {;
		var b = 1;
	};
	return a * b;
};

Now, let's test those 2 functions:

console.log('multiply')
console.log(multiply(5, 2))
console.log(multiply(5))

console.log('multiply_async:')
multiply_async(5, 2).then(console.log)
multiply_async(5).then(console.log)

In Chrome and Node, the output is:

multiply:
10
5
multiply_async:
10
5

But in Safari, the output is:

multiply:
10
5
multiply_async:
5
5

This is due to redeclaring b (var b = 1;), when b was already defined as the parameter of the function. In normal written Javascript, we would just reassign (b = 1).

Someone else has noticed this before:
https://medium.com/@alsotang/a-hoisting-bug-in-the-async-function-in-safari-ba1ecc386b4a

Therefore, it is not safe to re-declare function parameters using var