JSON parse error: Cannot read property

Question

I have created some little jt code, but it gives me error

function Mind(){
    var request = "request";
    var reply = "reply";
    var words = '';

    this.Reply = function(){
        if(request == words.nouns[0].noun){
            reply = words.nouns[0].noun;
        }
        else
            reply = this.words.nouns[0].noun;
    }

    this.SetRequest = function(req){
        request = req;
    }

    this.GetReply = function(){
        return reply;
    }

    this.Parse = function(u){
        var xmlhttp = new XMLHttpRequest();
        var url = u;
        var result;
        xmlhttp.onreadystatechange = function() {
            if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
                words = JSON.parse(xmlhttp.responseText);
            }
        }
        xmlhttp.open("GET", url, true);
        xmlhttp.send();
        return result;
    }

    this.Construct = function(){
        words = this.Parse('mind/words.json');
    }}

    var mind = new Mind();
    mind.Parse('mind/words.json');

and here is my json file

{
    "nouns": [
        {"noun": "child"},
        {"noun": "father"}
    ]
}

In command live all goes well, but when I run this code, appears error

Uncaught TypeError: Cannot read property 'nouns' of undefined

Also see Stackoverflow FAQ : [Access / process objects, arrays or JSON](http://stackoverflow.com/questions/11922383/access-process-nested-objects-arrays-or-json) — Yogi, May 11 '15 at 21:00

score 4 · Accepted Answer · edited May 23 '17 at 12:29

Mutliple errors. The most fundamental one is that your code ignores that XMLHttpRequest is async, and wont return a value in the same way as "regular" functions. Read about it here: How to make a function wait until a callback has been called using node.js. The TL;DR is that you have to pass in a "callback-function" to your parse-method and "return" your value using that function, instead of using a return-statement. Google for "javascript callbacks" and read a few tutorials if this concept is new to you!

You also have some minor errors, like returning result from Parse, but never actually setting result to anything. Also words is being assigned in multiple places in a way that doesn't really make sense. But both of these things will go away when you solve the sync/async issues.

EDIT:

Essentially the fix looks like this:

this.Parse = function(u, callback){ // this "callback" is new
    var xmlhttp = new XMLHttpRequest();
    var url = u;
    var result;
    xmlhttp.onreadystatechange = function() {
        if (xmlhttp.readyState == 4 && xmlhttp.status == 200) {
            words = JSON.parse(xmlhttp.responseText);
            callback(null, words); // we're "returning" the words here
        }
    }
    xmlhttp.open("GET", url, true);
    xmlhttp.send();
    // no return statement here!
}

this.Construct = function(){
    this.Parse('mind/words.json', function(error, words) {
        // here you can use your words!
    });
}}

Hi @Jakob . Please can you tell me more , how can I fix it ? Thank you — Gor, May 11 '15 at 18:06
there is an error Uncaught TypeError: callback is not a function — Gor, May 11 '15 at 18:41

JSON parse error: Cannot read property

1 Answers1