?= violates the type safety of await

[alray2569]

The await operator was designed to be type safe. That is, the await operator can operate on values of any type. When given a value that is not asynchronous, it returns its operand unchanged. For example, given the following:

async function get4Async() { return 4; }
function get4Promise() { return Promise.resolve(4); }
function get4Sync() { return 4; }

The following variables will all have the value of 4:

const n1 = await get4Async();
const n2 = await get4Promise();
const n3 = await (get4Promise());
const n4 = await get4Sync();
const n5 = await (get4Sync());
const n6 = await (Promise.resolve(4));
const n7 = await 4;

However, this does not work with the ?= polyfill:

const n1 ?= await get4Async();          // good
const n2 ?= await get4Promise();        // TypeError! get4Promise[Symbol.result] is not a function!
const n3 ?= await (get4Promise());      // good
const n4 ?= await get4Sync();           // TypeError! get4Sync[Symbol.result] is not a function!
const n5 ?= await (get4Sync());         // TypeError! (get4Sync())[Symbol.result] is not a function!
const n6 ?= await (Promise.resolve(4)); // good
const n7 ?= await 4;                    // TypeError! 4[Symbol.result] is not a function!

Even though all of these work with regular assignment, four of them throw a TypeError when used with "safe" assignment. Note of particular concern that the extra parentheses in n3, which are redundant with normal assignment, are now required with safe assignment!

[arthurfiorette]

We will not use the ?= syntax anymore.

#4

I just need more spare time to be able to rewrite it :)

[arthurfiorette]

Also, why this example would throw TypeError?

const n2 ?= await get4Promise();        // TypeError! get4Promise[Symbol.result] is not a function!

get4Promise is a function and thus would have Symbol.result in its prototype.

[alray2569]

@arthurfiorette Because the proposal said specifically that async functions would define Symbol.result. It did not say that all functions would define Symbol.result. Since get4Promise was not defined as an async function, it would therefore not have Symbol.result.