JS async/await - why does await need async?

Copied from https://stackoverflow.com/a/41744179/1483977 by @phaux:

These answers all give valid arguments for why the async keyword is a good thing, but none of them actually mentions the real reason why it had to be added to the spec.

The reason is that this was a valid JS pre-ES7

function await(x) {
  return 'awaiting ' + x
}

function foo() {
  return(await(42))
}

According to your logic, would foo() return Promise{42} or "awaiting 42"? (returning a Promise would break backward compatibility)

So the answer is: await is a regular identifier and it's only treated as a keyword inside async functions, so they have to be marked in some way.

Fun fact: the original spec proposed more lightweight function^ foo() {} for async syntax.

I'm not privy to the JavaScript language design discussions, but I assume it's for the same reasons that the C# language requires async (also see my blog).

Namely:

1. Backwards compatibility. If await was suddenly a new keyword everywhere, then any existing code using await as a variable name would break. Since await is a contextual keyword (activated by async), only code that intends to use await as a keyword will have await be a keyword.
2. Easier to parse. async makes asynchronous code easier to parse for transpilers, browsers, tools, and humans.