如何判断对象是否为Promise? [英] How do I tell if an object is a Promise?
问题描述
无论是ES6 Promise还是蓝鸟Promise,Q Promise等等。
Whether it's an ES6 Promise or a bluebird Promise, Q Promise, etc.
如何测试给定对象是否为Promise?
How do I test to see if a given object is a Promise?
推荐答案
承诺库如何决定
如果它有 .then
function - 这是仅标准承诺库使用的。
How a promise library decides
If it has a .then
function - that's the only standard promise libraries use.
Promises / A +规范有一个名为然后
的概念,它基本上是一个<$ c的对象$ c>然后方法。承诺将并且应该使用then方法同化任何。你提到的所有承诺都是这样做的。
The Promises/A+ specification has a notion called then
able which is basically "an object with a then
method". Promises will and should assimilate anything with a then method. All of the promise implementation you've mentioned do this.
如果我们看一下规范:
2.3.3.3 if
then
是一个函数,用x调用它,第一个参数resolvePromise,第二个参数rejectPromise
2.3.3.3 if
then
is a function, call it with x as this, first argument resolvePromise, and second argument rejectPromise
它还解释了它的基本原理这个设计决定:
It also explains the rationale for this design decision:
这种处理
然后
ables允许承诺实现互操作,只要他们公开一个Promises / A +兼容的然后
方法。它还允许Promises / A +实现用合理的方法同化不一致的实现。
This treatment of
then
ables allows promise implementations to interoperate, as long as they expose a Promises/A+-compliantthen
method. It also allows Promises/A+ implementations to "assimilate" nonconformant implementations with reasonable then methods.
你应如何决定
您不应该 - 而是调用 Promise.resolve(x)
( Q(x)
在Q中)始终将任何值或外部然后
转换为可信承诺。它比自己执行这些检查更安全,更容易。
How you should decide
You shouldn't - instead call Promise.resolve(x)
(Q(x)
in Q) that will always convert any value or external then
able into a trusted promise. It is safer and easier than performing these checks yourself.
你可以随时通过< a href =https://github.com/promises-aplus/promises-tests =noreferrer>测试套件:D
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