Generator.prototype.return()

¥Syntax

generatorInstance.return()
generatorInstance.return(value)

¥Parameters

value Optional

要返回的值。

¥Return value

Object 有两个属性：

¥An Object with two properties:

done

布尔值：

• 如果生成器函数的控制流已到达末尾，则为 true。
• false 如果生成器函数的控制流尚未到达末尾并且可以产生更多值。仅当 return 被捕获在 try...finally 中并且 finally 块中有更多 yield 表达式时，才会发生这种情况。

value

作为参数给出的值，或者，如果 yield 表达式包含在 try...finally 中，则为从 finally 块产生/返回的值。

¥Description

调用 return() 方法时，可以将其视为在生成器主体中当前挂起位置插入了一条 return value; 语句，其中 value 是传递给 return() 方法的值。因此，在典型的流程中，调用 return(value) 将返回 { done: true, value: value }。但是，如果 yield 表达式包含在 try...finally 块中，则控制流不会退出函数体，而是进入 finally 块。在这种情况下，如果 finally 块中有更多 yield 表达式，则返回的值可能会不同，并且 done 甚至可能是 false。

¥The return() method, when called, can be seen as if a return value; statement is inserted in the generator's body at the current suspended position, where value is the value passed to the return() method. Therefore, in a typical flow, calling return(value) will return { done: true, value: value }. However, if the yield expression is wrapped in a try...finally block, the control flow doesn't exit the function body, but proceeds to the finally block instead. In this case, the value returned may be different, and done may even be false, if there are more yield expressions within the finally block.

¥Examples

¥Using return()

以下示例显示了一个简单的生成器和 return 方法。

¥The following example shows a simple generator and the return method.

function* gen() {
  yield 1;
  yield 2;
  yield 3;
}

const g = gen();

g.next(); // { value: 1, done: false }
g.return("foo"); // { value: "foo", done: true }
g.next(); // { value: undefined, done: true }

如果在已经处于 "completed" 状态的生成器上调用 return(value)，生成器将保持在 "completed" 状态。

¥If return(value) is called on a generator that is already in "completed" state, the generator will remain in "completed" state.

如果未提供参数，则返回对象的 value 属性将为 undefined。如果提供了参数，它将成为返回对象的 value 属性的值，除非 yield 表达式封装在 try...finally 中。

¥If no argument is provided, the value property of the returned object will be undefined. If an argument is provided, it will become the value of the value property of the returned object, unless the yield expression is wrapped in a try...finally.

function* gen() {
  yield 1;
  yield 2;
  yield 3;
}

const g = gen();
g.next(); // { value: 1, done: false }
g.next(); // { value: 2, done: false }
g.next(); // { value: 3, done: false }
g.next(); // { value: undefined, done: true }
g.return(); // { value: undefined, done: true }
g.return(1); // { value: 1, done: true }

¥Using return() with try...finally

如果 yield 表达式封装在 try...finally 块中，则只有生成器本身才能知道 return 方法已被调用的事实。

¥The fact that the return method has been called can only be made known to the generator itself if the yield expression is wrapped in a try...finally block.

当在 try 块内挂起的生成器上调用 return 方法时，生成器中的执行将继续到 finally 块 - 因为 try...finally 语句的 finally 块始终执行。

¥When the return method is called on a generator that is suspended within a try block, execution in the generator proceeds to the finally block — since the finally block of try...finally statements always executes.

function* gen() {
  yield 1;
  try {
    yield 2;
    yield 3;
  } finally {
    yield "cleanup";
  }
}

const g1 = gen();
g1.next(); // { value: 1, done: false }

// Execution is suspended before the try...finally.
g1.return("early return"); // { value: 'early return', done: true }

const g2 = gen();
g2.next(); // { value: 1, done: false }
g2.next(); // { value: 2, done: false }

// Execution is suspended within the try...finally.
g2.return("early return"); // { value: 'cleanup', done: false }

// The completion value is preserved
g2.next(); // { value: 'early return', done: true }

// Generator is in the completed state
g2.return("not so early return"); // { value: 'not so early return', done: true }

finally 块的返回值也可以成为 return 调用返回结果的 value。

¥The return value of the finally block can also become the value of the result returned from the return call.

function* gen() {
  try {
    yield 1;
  } finally {
    return "cleanup";
  }
}

const g1 = gen();
g1.next(); // { value: 1, done: false }
g1.return("early return"); // { value: 'cleanup', done: true }

¥See also

• function*