为什么下面的_.map函数什么都不返回？

Question

I created a function to find objects:

store.js

store.find = () => {
  const query = new Parse.Query(Document)
  return query.find()
}

That I'm using like this (and assigning the value to this.documents ):

main.js

store.find().then((results) => {
  this.documents = _.map(results, (result) => {
    return result.toJSON()
  })
})

It works. But the store.find().then( ...) part is repeated many times in main.js so I turned it into a function:

store.js

store.fetch = (documents) => {
  return store.find().then((results) => {
    documents = _.map(results, (result) => {
      return result.toJSON()
    })

  })
}

Which I use like this:

main.js

store.fetch(this.documents)

But nothing is being assigned to this.documents and there are no error messages. What's could be the problem?

NOTE: this.documents is an array of objects. Example:

[{
  title: this.title,
  content: this.content,
}, ...

EDIT:

I did this:

store.fetch = (documents) => {
  store.find().then((results) => {
    documents = _.map(results, (result) => {
      return result.toJSON()
    })
    console.log(documents)
  })
}

And documents is being assigned:

[Object, Object, Object, ...]

So I think it's just not assigning this array to this.documents . Maybe variables can't be arguments and be assigned with a value at the same time?

Answer 1

You are passing this.documents as an argument and attempting to modify that property inside the function. However, that can't work, because Javascript does not have "pass by reference" . Only "pass by value".

Instead, you can try passing the object and the property name separately:

store.fetch = (obj, prop) => {
  return store.find().then((results) => {
    obj[prop] = _.map(results, (result) => {
      return result.toJSON()
    })
  })
}
store.fetch(this, 'documents')

Answer 2

Try to get your results in a .then block after store.fetch(this.documents) . And don't forget to store context

var self = this;
store.fetch(self.documents).then(function () {
    console.log(self.documents);
});

Or usign es6

store.fetch(this.documents).then(() => {
    console.log(this.documents);
});