This isn't the only bug/bad practice that the git-copilot page shows off. Here's the memoization for js suggestion on the main page:

  const memoize = fn => {
    const cache = {};
    return (...args) => {
      const key = JSON.stringify(args);
      return (cache[key] = cache[key] || fn(...args));
    };
  }

It uses js falsyness to figure out whether it can return from the cache or if it needs to invoke the wrapped function. However, js falsy is pretty dangerous. "cache[key]" will return undefined if there's no value in the cache for those arguments, but undefined is not the only falsy value. Here's the full list: https://developer.mozilla.org/en-US/docs/Glossary/Falsy

Many of those values are reasonable function return values meaning your cache will simply not work for some function outputs.

The key generation is also a little problematic. Stringifying the input may produce huge strings which are then kept in memory for an indefinite period of time which creates a memory leak.

Here's the bottom line on git co-pilot. It's a huge step forward and I think everyone is going to be using tools like it in the future. There's no doubt to me that it will make good programmers way more productive. However, not-so-good programmers will become way more destructive since copilot will let them write bad, unoptimized code faster than every before.