问题
Is the code below guaranteed to output HERE
?
var p = new Promise(() => console.log("HERE"))
(That is, does var p = new Promise(fn)
always execute fn
if p.then(…)
is never called to do something with the result?)
More specifically, in the context of service workers, if I call Cache.delete() but never call .then()
on the return value (or I throw away the return value), is the cache entry guaranteed to be deleted?
回答1:
Yes, it is guaranteed. The specification of Promise has this step which will always be evaluated:
- Let completion be Call(executor, undefined, «resolvingFunctions.[[Resolve]], resolvingFunctions.[[Reject]]»).
where executor
is what you passed to the Promise
constructor, and Call results in that code being run. This all happens before the Promise
is even returned to your p
variable.
回答2:
As James said, it is guaranteed that the function will be called. Though this doesn't guarantee that the cache entry gets deleted!
You have to check the value of the promise resolution (true
if the cache entry is deleted, false
otherwise).
来源:https://stackoverflow.com/questions/35177230/are-promises-lazily-evaluated