Question

当省略参数时，我无法使用Bluebird library的promisifyAll函数来处理ES2015类的对象实例方法。我正在使用Node 7.8.0。这是一些示例代码：

// dog.js
class Dog {
  constructor(opts) {
    this.name = opts.name || 'Fido';
  }

  fetchBone(bone, callback) {
    console.log('fetching the bone');
    callback(null, 'got it');
  }
}

exports = module.exports = Dog;

假设骨骼是fetchBone的可选参数。当我通过它时，一切都很好。

> var Dog = require('./dog');
> Promise = require('bluebird');
> Promise.promisifyAll(Dog); // omitting this line doesn't help
> Promise.promisifyAll(Dog.prototype);
> var puppy = new Dog();
> puppy.fetchBoneAsync('juicy bone')
       .then(result => console.log('from promise:', result))
       .catch(err => console.error('failure:', err));
fetching the bone
Promise {
  _bitField: 0,
  _fulfillmentHandler0: undefined,
  _rejectionHandler0: undefined,
  _promise0: undefined,
  _receiver0: undefined }
> from promise: got it

当我没有传入骨头时失败。

> puppy.fetchBoneAsync()
       .then(result => console.log('from promise:', result))
       .catch(err => console.error('failure:', err));
fetching the bone
Promise {
  _bitField: 0,
  _fulfillmentHandler0: undefined,
  _rejectionHandler0: undefined,
  _promise0: undefined,
  _receiver0: undefined }
> failure: TypeError: callback is not a function
    at Dog.fetchBone (dog.js:8:5)
    at Dog.tryCatcher (node_modules/bluebird/js/release/util.js:16:23)
    at Dog.ret [as fetchBoneAsync] (eval at makeNodePromisifiedEval (node_modules/bluebird/js/release/promisify.js:184:12), <anonymous>:14:23)
    at repl:1:7
    at ContextifyScript.Script.runInThisContext (vm.js:23:33)
    at REPLServer.defaultEval (repl.js:339:29)
    at bound (domain.js:280:14)
    at REPLServer.runBound [as eval] (domain.js:293:12)
    at REPLServer.onLine (repl.js:536:10)
    at emitOne (events.js:101:20)
    at REPLServer.emit (events.js:191:7)
    at REPLServer.Interface._onLine (readline.js:241:10)
    at REPLServer.Interface._line (readline.js:590:8)
    at REPLServer.Interface._ttyWrite (readline.js:869:14)
    at REPLServer.self._ttyWrite (repl.js:609:7)
    at ReadStream.onkeypress (readline.js:120:10)

奇怪的是，如果我传入undefined骨骼，它就会起作用。

> puppy.fetchBoneAsync(undefined)
       .then(result => console.log('from promise:', result))
       .catch(err => console.error('failure:', err));
fetching the bone
Promise {
  _bitField: 0,
  _fulfillmentHandler0: undefined,
  _rejectionHandler0: undefined,
  _promise0: undefined,
  _receiver0: undefined }
> from promise: got it

有人知道这里发生了什么吗？

Answer 1

是的，这是正确的行为。承诺映射到原始函数。所以fetchBone()在第一个调用中接收两个参数，在第二个示例中接收一个。

这意味着在第二个示例中，调用fetchBone()时callback未定义且bone本身是由promisification创建的回调函数来处理promise本身。

省略参数会导致ES2015类的默认函数失败

1 个答案: