Promise A+规范（译）

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前言

试着翻译下ECMA中对Promise实现规范，顺便学习下Promise（承诺）。

译文

原文地址 Promise/A+

ok，开始一段一段的翻译

An open standard for sound, interoperable JavaScript promises—by implementers, for implementers.

译：一个完整开放的，可互相操作的JavaScript Promise的开放标准，由实现者指定，面向实现者。

A promise represents the eventual result of an asynchronous operation. The primary way of interacting with a promise is through its then method, which registers callbacks to receive either a promise’s eventual value or the reason why the promise cannot be fulfilled.

译：一个promise意味着一个不是现在发生（非共时）的操作的最终结果。与一个promise进行交流的基本的方式是通过它的then方法，then方法注册了一个的回调函数，这个函数要么接收Promise的最终值，要么接收为什么这个promise不能转为完成状态（fulfilled）的原因。

This specification details the behavior of the then method, providing an interoperable base which all Promises/A+ conformant promise implementations can be depended on to provide. As such, the specification should be considered very stable. Although the Promises/A+ organization may occasionally revise this specification with minor backward-compatible changes to address newly-discovered corner cases, we will integrate large or backward-incompatible changes only after careful consideration, discussion, and testing.

译：规范详细说明了then方法的的行为，提供了一个可以互相操作的基础来规定行为，所有和Promise/A+规范一致的promise实现依赖这个基础。因此，规范应该被认为是非常稳定的。虽然Promise/A+组织可能偶尔修改这份规范中少数的可以向后兼容的变化来解决新发现的极端情况，但是只有在经过深思熟虑，讨论和测试后才会合并大型的或者不能向后兼容的变化。

Historically, Promises/A+ clarifies the behavioral clauses of the earlier Promises/A proposal, extending it to cover de facto behaviors and omitting parts that are underspecified or problematic.

译：历史上，Promise/A+阐明了早期Promise/A提案中的行为条文，并扩展到覆盖实际上的行为和忽略未指定和有问题的部分。

Finally, the core Promises/A+ specification does not deal with how to create, fulfill, or reject promises, choosing instead to focus on providing an interoperable then method. Future work in companion specifications may touch on these subjects.

译：最后，核心的Promise/A+规范没有处理如何创建，完成，或者拒绝Promise，而是专注于提供一个可以互相操作的then方法，在未来的配套规范中会涉及这些问题。

1. Terminology（术语）

  • ‘promise’ is an object or function with a then method whose behaior conforms to this specification.
  • ‘thenable’ is an object or function that defines a then method.
  • ‘value’ is any legal JavaScript value (including undefined, a thenable, or a promise).
  • ‘exception’ is a value that is thrown using the throw statement.
  • ‘reason’ is a value that indicates why a promise was rejected.

译：

  • “promise”应该是一个带有then方法的对象或者函数，这个then方法的行为符合这份规范。
  • “thenable”是一个定义了then方法的对象或者函数。
  • “value”可以是任何合法的JavaScript的值（包括undefined，一个thenable或者一个promise）。
  • “exception”是一个使用了异常语句所抛出的值。
  • “reason”是一个表示为什么promise被拒绝的值。

2. Requirements（要求）

2.1 Promise States（Promise的状态）

A promise must be in one of three states: pending, fulfilled, or rejected.

译：一个Promise的状态必须是等待状态，完成状态，或者拒绝状态中的一个。

  • When pending, a promise:
    • may transition to either the fulfilled or rejected state.
  • When fulfilled, a promise:
    • must not transition to any other state.
    • must have a value, which must not change.
  • When rejected, a promise:
    • must not transition to any other state.
    • must have a reason, which must not change.

译：

  • 当promise处于等待状态的时候：
    • promise可以转变成完成状态或者拒绝状态的其中一个状态。
  • 当promise处于完成状态的时候：
    • 不能转变成其他任何状态。
    • 必须有一个value，并且这个value不能改变。
  • 当promise处于拒绝状态的时候：
    • 不能转变成其他任何状态。
    • 必须有一个reason，并且这个reason不能改变。

Here, “must not change” means immutable identity (i.e. ===), but does not imply deep immutability.

译：这里的”不能改变”的意思是特征不变（即严格相等 === ），但不是意味着深层次的不变。（意思应该是浅相等，但不是深相等）

2.2 The then Method（then方法）

A promise must provide a then method to access its current or eventual value or reason.

译：一个promise必须提供一个then方法来使用它当前或者最终的值或者（拒绝状态的）原因。

A promise’s then method accepts two arguments:

译：promise的then方法接收两个参数

promise.then(onFulfilled, onRejected)

  • Both onFulfilled and onRejected are optional arguments:
    • If onFulfilled is not a function, it must be ignored.
    • If onRejected is not a function, it must be ignored.

译：

  • onFulfilled和onRejected参数都是可选的的参数:
    • 如果onFulfilled不是一个函数，则将被忽略。
    • 如果onRejected不是一个函数，则将被忽略。
  • If onFulfilled is a function:
    • it must be called after promise is fulfilled, with promise‘s value as its first argument.
    • it must not be called before promise is fulfilled.
    • it must not be called more than once.

译：

  • 如果onFulfilled是一个函数：
    • 在promise完成之后调用，并且函数的第一个参数为promise的value。
    • 在promise完成之前不能被调用。
    • 不能被调用超过一次。
  • If onRejected is a function:
    • it must be called after promise is rejected, with promise’s reason as its first argument.
    • it must not be called before promise is rejected.
    • it must not be called more than once.

译：

  • 如果onRejected是一个函数
    • 在promise被拒绝之后调用，并且函数的第一个参数为promise被拒绝的原因。
    • 在promise被拒绝之前不能被调用。
    • 不能被调用超过一次。

onFulfilled or onRejected must not be called until the execution context stack contains only platform code.#3.1

译：在执行上下文的堆栈中仅包含平台代码之前，onFulfilled和onRejected不能被调用。

  • onFulfilled and onRejected must be called as functions (i.e. with no this value).#3.2

译：

  • onFulfilled和onRejected函数必须以函数方式调用（即不能是this这个值）。
  • then may be called multiple times on the same promise.
    • If/when promise is fulfilled, all respective onFulfilled callbacks must execute in the order of their originating calls to then.
    • If/when promise is rejected, all respective onRejected callbacks must execute in the order of their originating calls to then.

译：

  • 在同一个promise中then方法可以重复调用。
    • 如果或者说当promise完成时，各自的onFulfilled回调函数必须按照它们最初在then方法上调用的顺序执行。
    • 如果或者说当promise被拒绝时，各自的onRejected回调函数必须按照它们最初在then方法上调用的顺序执行。
  • then must return a promise.#3.3
    promise2 = promise1.then(onFulfilled, onRejected);
  • If either onFulfilled or onRejected returns a value x, run the Promise Resolution Procedure [[Resolve]](promise2, x).
    • If either onFulfilled or onRejected throws an exception e, promise2 must be rejected with e as the reason.
    • If onFulfilled is not a function and promise1 is fulfilled, promise2 must be fulfilled with the same value as promise1.
    • If onRejected is not a function and promise1 is rejected, promise2 must be rejected with the same reason as promise1.

译：

  • then方法必须返回一个promise
    promise2 = promise1.then(onFulfilled, onRejected);
  • 如果onFulfilled或者onRejected其中一个函数返回了一个值x，那么执行Promise的解决过程[[Resolve]](promise2,x)
    • 如果onFulfilled或者onRejected其中一个函数抛出了一个异常e，那么promise2必须为以e为原因的拒绝状态。
    • 如果onFulfilled不是一个函数而且promise1为完成态时，promise2必须为完成态并且带有和promise1相同的值
    • 如果onRejected不是一个函数而且promise1为拒绝态时，promise2必须为拒绝态并且带有和promise1相同的原因。

2.3 The Promise Resolution Procedure（Promise的解决程序）

The promise resolution procedure is an abstract operation taking as input a promise and a value, which we denote as [[Resolve]](promise, x). If x is a thenable, it attempts to make promise adopt the state of x, under the assumption that x behaves at least somewhat like a promise. Otherwise, it fulfills promise with the value x.

译：promise解决过程是一个输入一个promise或者一个值抽象的操作，这个操作我们用[[Resolve]](promise, x)这个标记来表示。如果x是一个thenable对象的话，在x的行为至少有点像一个promise的假设下，这个方法尝试将promise采用x的状态，否则，这个方法将以x的值来完成promise。

This treatment of thenables allows promise implementations to interoperate, as long as they expose a Promises/A+-compliant then method. It also allows Promises/A+ implementations to “assimilate” nonconformant implementations with reasonable then methods.

译：对于thenables的论述允许promise的实现可以相互操作，只要它们暴露了一个Promise/A+一致性的then方法。它也允许Promise/A+实现以合理的then方法来”同化”不一致的实现。

To run [[Resolve]](promise, x), perform the following steps:

译：为了执行[[Resolve]](promise, x)，需要履行以下步骤：

  • If promise and x refer to the same object, reject promise with a TypeError as the reason.

译：

  • 如果promise和x是同一个对象的话，以TypeError为原因拒绝这个promise。
  • If x is a promise, adopt its state#3.4:
    • If x is pending, promise must remain pending until x is fulfilled or rejected.
    • If/when x is fulfilled, fulfill promise with the same value.
    • If/when x is rejected, reject promise with the same reason.

译：

  • 如果x是一个promise，采用它的状态：
    • 如果x即将发生，promise必须保持即将发生的状态直到x完成或者被拒绝。
    • 如果或者当x完成，使得这个promise以相同的值完成
    • 如果或者当x被拒绝，使得这个promise以相同的原因被拒绝。
  • Otherwise, if x is an object or function,
    • Let then be x.then.#3.5
    • If retrieving the property x.then results in a thrown exception e, reject promise with e as the reason.
    • If then is a function, call it with x as this, first argument resolvePromise, and second argument rejectPromise, where:
      • If/when resolvePromise is called with a value y, run [[Resolve]](promise, y).
      • If/when rejectPromise is called with a reason r, reject promise with r.
      • If both resolvePromise and rejectPromise are called, or multiple calls to the same argument are made, the first call takes precedence, and any further calls are ignored.
      • If calling then throws an exception e,
        • If resolvePromise or rejectPromise have been called, ignore it.
        • Otherwise, reject promise with e as the reason.
    • If then is not a function, fulfill promise with x.

译：

  • 否则，如果x是一个对象或者函数，
    • 设置then方法为x.then
    • 如果检索x.then的属性导致一个抛出的异常e的话，以异常e为原因拒绝promise。
    • 如果then是一个函数，以x为this(上下文)的方式调用，第一个参数为resolvePromise，第二个参数为rejectPromise：
      • 如果或者当resolvePromise以一个值y被调用，执行[[Resolve]](promise,y)。
      • 如果或者当rejectPromise以一个原因r被调用，以r来拒绝这个promise。
      • 如果resolvePromise和rejectPromise都被调用，或者对同一个参数进行了多次的调用，只有第一次调用生效，其他即将的调用被忽略。
      • 如果执行then抛出了一个异常e
        • 如果resolvePromise或者rejectPromise已经被调用，则忽略它。
        • 否则，以e作为原因拒绝这个promise。
    • 如果then不是一个函数，则以x为值完成promise。
  • If x is not an object or function, fulfill promise with x.

译：

  • 如果x不是一个对象或者函数，以x为值完成promise。

If a promise is resolved with a thenable that participates in a circular thenable chain, such that the recursive nature of [[Resolve]](promise, thenable) eventually causes [[Resolve]](promise, thenable) to be called again, following the above algorithm will lead to infinite recursion. Implementations are encouraged, but not required, to detect such recursion and reject promise with an informative TypeError as the reason.#3.6

译：如果一个promise通过一个thenable解决，而这个thenable参与了一个环形的thenable链，以至于[[Resolve]](promise, thenable)的递归性质最终造成[[Resolve]](promise, thenable)被再一次的调用，以上的计算将会导致无限的递归。实现应该鼓励去发现这样的递归然后提供一个TypeError错误作为原因来拒绝promise，但这并不是规定的。

3.Notes（一些地方的解释）

3.1

Here “platform code” means engine, environment, and promise implementation code. In practice, this requirement ensures that onFulfilled and onRejected execute asynchronously, after the event loop turn in which then is called, and with a fresh stack. This can be implemented with either a “macro-task” mechanism such as setTimeout or setImmediate, or with a “micro-task” mechanism such as MutationObserver or process.nextTick. Since the promise implementation is considered platform code, it may itself contain a task-scheduling queue or “trampoline” in which the handlers are called.

译：这里的”平台代码”意思是指引擎，环境和promise实现的代码。实际上，这个要求确保onFulfilled和onRejected函数在then方法调用的事件循环之后以一个新的堆栈异步执行。可以以宏任务机制比如setTimeout或者setImmediate来实现，又或者以微任务机制比如MutationObserver或者process.nextTick来实现。由于promise实现被认为是平台代码，它自身可能包含一个调用处理程序的任务调度队列或者”蹦床”。

3.2

That is, in strict mode this will be undefined inside of them; in sloppy mode, it will be the global object.

译：在严格模式下this在内部将会是undefined，而在非严格模式下，它将会是全局对象。

3.3

Implementations may allow promise2 === promise1, provided the implementation meets all requirements. Each implementation should document whether it can produce promise2 === promise1 and under what conditions.

译：假设实现满足所有要求，实现可能允许promise2 === promise1。每个实现应该记录在什么情况下是否产生promise2 === promise1。

3.4

Generally, it will only be known that x is a true promise if it comes from the current implementation. This clause allows the use of implementation-specific means to adopt the state of known-conformant promises.

译：一般地，只有在promise来自当前（自身）的实现的情况才能知道x是一个真的promise。这个条文允许具体实现的方式去采用已知一致的promise。

3.5

This procedure of first storing a reference to x.then, then testing that reference, and then calling that reference, avoids multiple accesses to the x.then property. Such precautions are important for ensuring consistency in the face of an accessor property, whose value could change between retrievals.

译：程序的第一步储存x.then的引用，然后检验引用，然后调用引用，避免多次获取x.then的属性。这样的预防对于确保那些可能在检索过程中变化的存储属性的一致性很重要。

3.6

Implementations should not set arbitrary limits on the depth of thenable chains, and assume that beyond that arbitrary limit the recursion will be infinite. Only true cycles should lead to a TypeError; if an infinite chain of distinct thenables is encountered, recursing forever is the correct behavior.

译：实现不应该对thenable链的深度设置任意的限制，并假设超过这个任意的限制递归将是无限的。只有正确的循环应该导致一个TypeError的错误；如果遇到一个不同的thenables组成的无限的链，永久的递归才是正确的行为。