定制容器：我可以使用“ then”方法创建自己的对象吗？

Question

I'm wondering if custom "thenables" (aka an object with a .then() method) are approved/documented? How do they compare to real Promises? I suppose it matters how you implement then, so I'm wondering if there's documentation with some DOs and DONTs.

This page suggests:

.then may return an arbitrary “thenable” object, and it will be treated the same way as a promise.

Can't find any docs on this.

Answer 1

How do thenables compare to real Promises?

In that you don't know whether they are real promises or not. Have a look at Regarding Promises/A+ Specification, what is the difference between the terms "thenable" and "promise"? .

I'm wondering if there's documentation with some DOs and DONTs about how to implement them (as that seems to be what matters)

There's the (pretty simple) Promises/A+ specification that documents how thenables are treated. ES6 promises (and by extension, await ) follow this. Basically:

• your object has a property with the name then whose value is a function
• the function will get called with two callbacks
• you can call either of them - asynchronously or not
• the first call determines what happens to the promise that assimilated your thenable

It's really no magic. You call the first argument when you want to resolve with a value, and you call the second argument when you want to reject. There are no DONTs assuming a proper promise implementation - you can call the callbacks as often as you want, and keep them around as long as you want: the calls should be ignored and the reference should not leak memory.