Asynchronous "do expressions"

Overview

Building on async functions and the not-formally-proposed "do expressions", "async do expressions" let you write asynchronous blocks in an expression context, returning the completion value (a Promise).

This is useful when functions expect an asynchronous result (a Promise) as input, and the steps to produce that result can be expressed in line with the function call.

Example:

fetchEvent.respondWith(async {
  var response = await fetch('https://example.com/resource');
  var body = await response.text();
  'BEGIN---\n' + body + '\n---END\n';
});

This is similar to an "immediately invoked async function expression", i.e.:

fetchEvent.respondWith((async () => {
  var response = await fetch('https://example.com/resource');
  var body = await response.text();
  return 'BEGIN---\n' + body + '\n---END\n';
})());

Or more verbosely:

async function f() {
  var response = await fetch('https://example.com/resource');
  var body = await response.text();
  return 'BEGIN---\n' + body + '\n---END\n';
};
let p = f();
fetchEvent.respondWith(p);

Syntax

PrimaryExpression :
  ...
  AsyncBlockExpression
  
AsyncBlockExpression :
  "async" [no LineTerminator here] Block
  
ExpressionStatement :
  [lookahead ∉ {{, function, class, let [, async}] Expression;

Semantics

TODO - static and runtime semantics

• var hoisting behavior needs definition
• break, continue and return need definition

References

• async functions
• do expressions strawman
• TC39 meeting notes: Do Expression
• es-discuss thread: monadic extension to do-notation