未从函数“返回”未分配给变量

Question

I have written a function that searches recursively through an object to get all specific elements out of it, and return the resulting array of elements.

The function itself works perfectly, but for some reason the result of the "return" portion does not get assigned to the variable that called the function.

This is the function itself:

var getLinksInObject = function(object){
    // Define Variables
    var allLinks = [];
    var getLinksTimeout = null;

    // Function to get the links in an object. Called recursively
    var getLinks = function(object){
        // Get category and link links
        var categories = object.categories;
        var links = object.links;

        // If there are links
        if (links && links.length > 0){
            // Push each of them to the allLinks array
            for (var i=0, j=links.length; i<j; i++){
                allLinks.push(links[i]);
            };
        };

        // If there are category links, push them to the allLinks array
        if (categories && categories.length > 0){
            for (var i=0, j=categories.length; i<j; i++){
                // Build the link
                var link = {
                    title: categories[i].title,
                    link: categories[i].link
                };
                // Push the link to allLinks
                allLinks.push(link);

                // If there are sub-links, run getLinks on that object to get the next level of links
                if (categories[i].categories) {
                    // Reset the listener so it knows we are going for another round
                    resetTimeout();

                    // Get the links in the next level
                    getLinks(categories[i]);
                }           
            };
        };
    };

    // Listens for when the recursive calls finish and returns the result
    var resetTimeout = function(){
        clearTimeout(getLinksTimeout);
        getLinksTimeout = setTimeout(function(){
            log(allLinks);
            return allLinks;
        }, 50);
    };

    resetTimeout();
    getLinks(object);
};

This is a test object to run it against that matches the structure of the actual object:

{
    link: "http://test.com",
    title: "This is a test",
    categories: [
        {
            link: "http://test.com",
            title: "This is a test",
            categories: [
                {
                    link: "http://test.com",
                    title: "This is a test",
                    categories: [
                        {

                        }
                    ]
                }
            ]
        },
        {
            link: "http://test.com",
            title: "This is a test",
            categories: [
                {
                    link: "http://test.com",
                    title: "This is a test",
                    categories: [
                        {
                            link: "http://test.com",
                            title: "This is a test",
                            categories: [
                                {
                                    link: "http://test.com",
                                    title: "This is a test"
                                }
                            ]
                        }
                    ]
                },
                {
                    link: "http://test.com",
                    title: "This is a test",
                    categories: [
                        {

                        }
                    ]
                },
                {
                    link: "http://test.com",
                    title: "This is a test",
                    categories: [
                        {

                        }
                    ]
                }
            ]
        },
        {
            link: "http://test.com",
            title: "This is a test",
            categories: [
                {
                    link: "http://test.com",
                    title: "This is a test",
                    categories: [
                        {

                        }
                    ]
                }
            ]
        }
    ]
}

And this is a fiddle that duplicates the problem. You'll be able to see the problem in the console as the logging of "result" gets logged out as undefined.

http://jsfiddle.net/KS7LG/

Answer 1

The function you assign to getLinksInObject doesn't have a return statement. It will always return undefined .

The only return statement in your entire code is this:

setTimeout(function(){
console.log(allLinks);
    return allLinks;
}, 50);

That function is being called by setTimeout , and setTimeout doesn't pay attention to any return values from the functions it calls.

setTimeout is asynchronous, it runs code later and doesn't block the function that called it from continuing. If you want to deal with data generated after the time runs out, then you have to do so going forwards from the function you pass, it is too late to pass data back.

Answer 2

Javascript (mostly) operates synchronously so you don't need any timeout related code to make your function work. Returning the value immediately as it is calculated will do fine.

See: jsFiddle

In particular, the following change:

              //resetTimeout(); <- remove this

                // Get the links in the next level
                getLinks(categories[i]);
            }           
        };
    };
};

//removed timeout code

getLinks(object);
return allLinks;

Answer 3

Return won't work with setTimeout as it's asynchronous. You'll need to create a callback to achieve what you want:

var getLinksInObject = function(object, callback) {
    // ...            
    var resetTimeout = function(){
        clearTimeout(getLinksTimeout);
        getLinksTimeout = setTimeout(function(){
            log(allLinks);
            callback(allLinks);
        }, 50);
    };

   // ...
};

Then you'll have to modify the getLinksInObject call:

getLinksInObject(function(allLinks) {
     // do what you need to do with allLinks here
});