javascript函数调用时间

Question

I am trying to implement a for loop that iterates through a list and subsequently calls two functions, only if the first function results are found.

The issue is that the second function (search.similar) might be taking longer to fetch results.

With the code below, when I run, all of the appropriate output from (search.locate) is correct, but only the last element's results from myList are stored from the (search.similar) function.

ie. all_results = [[cat_res1,mouse_res2],[dog_res1,mouse_res2],[mouse_res1,mouse_res2]]

How do I fix this to append the right results in the right order?

ie. all_results = [[cat_res1,cat_res2],[dog_res1,dog_res2],[mouse_res1,mouse_res2]]

var search = require('./search');
var myList = ['cat','dog','mouse'];
var all_results = [];
for (i=0; i<myList.length; i++){
  /* locate function*/
  search.locate(myList[i], function (err, searchResult){
    if (err){
      console.log("Error");
      return;
    }

    if (!searchResult){
      console.log("Cannot find it");
      return;
    }

    /*similarity function*/
    /* seems to take longer*/
    search.similar(myList[i], function (err, similarResult){
      if (err){
        return;
      }

      if (!similarResult){
        return;
      }

      var res1 = searchResult.data;
      var res2 = similarResult.data;
      /* append results to array*/
      all_results.push([res1,res2]);
    }
  });
}

Answer 1

Javascript can be thought of as asynchronous, in that the execution of particular functions do not necessarily happen synchronously, however "describing JavaScript as asynchronous is perhaps misleading. It's more accurate to say that JavaScript is synchronous and single-threaded with various callback mechanisms"

In order to accomplish your goal, though you may still get some ordering issues with the top array, you will need to wrap your .similar() call in another function that takes both arguments. Your reference to the "item" on the top search is changing:

function searchNestedSimilar(item, topRes) {
    search.similar(item, function (err, similarResult) {

        if (err){
            return;
        }
        if (!topRes){
            return;
        }

        var res1 = topRes.data
        var res2 = similarResult.data

        // append results to array
        all_results.push([res1,res2])

    }
} 

function searchLocate(item) {
   search.locate(item, function (err, searchResult) {

    if (err){
        console.log("Error");
        return;
    }
    if (!searchResult){
        console.log("Cannot find it");
        return;
    }
    searchNestedSimilar(item, searchResults);
}

I encapsulated both calls to keep it modular, but since "item" is in the closure, you really only need the searchLocate() function to wrap your capture your item reference during iteration.

Answer 2

This is a good case for Promises (see Bluebird JS for example http://bluebirdjs.com/docs/getting-started.html ) or you could do it with async.map().

This page talks about it well, too. http://promise-nuggets.github.io/articles/14-map-in-parallel.html

There are many Stack Overflows discussing Promises as well. Understanding JS Promises for example.

A rough example of how to write this with a Promise:

var search = require('./search');
var myList = ['cat','dog','mouse']
var all_results = []
var Promise = require('bluebird');
var locate = Promise.promisify(search.locate);
var similar = Promise.promisify(search.similar);

for (i = 0; i < myList.length; i++){
    // locate function
    locate(myList[i], function (err, searchResult) {
        if (err) {
            console.log("Error");
            return;
        }
        if (!searchResult){
            console.log("Cannot find it");
            return;
        }
    }).then(function(result) {
        //similarity function
        similar(myList[i], function (err, similarResult) {
            if (err){
                return;
            }
            if (!similarResult){
                return;
            }

            var res1 = searchResult.data
            var res2 = similarResult.data

            // append results to array
            all_results.push([res1,res2])
        }).finally(function() {
            // NOP
        });
    });
}