esbuild --bundle omits variable assignment if variable is redeclared

[pcattori]

esbuild version: 0.14.25

---

example.js:

var a = 1;
var a;
console.log(a);

Running node example.js prints 1. ✅

Running esbuild example.js --bundle --outfile=build.js produces:

(() => {
  // example.js
  var a;
  console.log(a);
})();

Note the missing a = 1 assignment in build.js.
Running node build.js prints undefined, when it should print 1. ❌

---

For context: the React development build hosted on esm.sh (https://esm.sh/react@17.0.2?dev) includes a variable re-declaration resulting in Uncaught TypeError: _a is undefined when running the built code.

[pcattori]

Possible duplicate of #2080 and #2098