226335 2021-05-27 09:10:33 -0700 Redeclaring an argument of the async function makes it `undefined` 2023-10-15 11:59:27 -0700 1 1 1 Unclassified WebKit JavaScriptCore Safari 14 All All RESOLVED DUPLICATE 223533 InRadar P2 Normal --- 1 kotkov webkit-unassigned ashvayka fpizlo webkit-bug-importer ysuzuki oldest_to_newest 1764346 0 kotkov 2021-05-27 09:10:33 -0700 Consider the following JavaScript code example: function foo(x) { console.log(x); var x = 2; console.log(x); } foo(1); async function bar(x) { console.log(x); var x = 2; console.log(x); } bar(1); Running this example in stable Chromium or Firefox versions produces the following expected result: 1 2 1 2 Running this example in Safari produces the following *unexpected* result: 1 2 undefined 2 This issue reproduces in Safari 12.x, 13.x, 14.x on macOS and iOS. This issue reproduces in Safari 14.1.1 on macOS 11.4. Apparently, redeclaring the argument `x` of an async function with `var x` in the function's body causes the initial argument to lose its value and become `undefined`. As shown in the above example, this behavior is specific to async functions and doesn't reproduce in non-async functions. Redeclaring function arguments in this way is a common technique used by code minifiers such as uglify-js, as that sometimes allows reusing shorter variable names. 1766412 1 webkit-bug-importer 2021-06-03 09:11:19 -0700 <rdar://problem/78818618> 1985202 2 ashvayka 2023-10-15 11:59:27 -0700 *** This bug has been marked as a duplicate of bug 223533 ***