Error encountered while attempting to parse JSON: Unable to access property

Question

Error encountered while attempting to parse JSON: Unable to access property

Help needed! I've written some code in JavaScript, but it's throwing an error.

function Brain(){
    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('brain/words.json');
    }}

    var brain = new Brain();
    brain.Parse('brain/words.json');

I am also providing my json file content below:

{
    "nouns": [
        {"noun": "dog"},
        {"noun": "cat"}
    ]
}

Everything works fine in the testing environment, but upon executing the code, an error is thrown:

Uncaught TypeError: Cannot read property 'nouns' of undefined

javascript json

Answer 1

Answer №1

Your code contains several errors, with the main one being that it does not account for the asynchronous nature of the XMLHttpRequest function. This means it will not return a value in the same way as synchronous functions. To learn more about this issue, you can refer to this resource: How to make a function wait until a callback has been called using node.js. In essence, you need to use a callback function in your parse method to handle the return value instead of relying on a traditional return statement. If this concept is new to you, I recommend researching "javascript callbacks" to understand it better.

Additionally, there are some minor issues in your code such as returning 'result' from 'Parse' without setting it to anything first, and assigning 'words' in multiple places in a confusing manner. These problems can be resolved once you address the synchronization and asynchrony issues.

EDIT:

The suggested fix involves modifying your code as follows:

this.Parse = function(u, callback){ // introducing the "callback" parameter
    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); // passing the words to the callback function
        }
    }
    xmlhttp.open("GET", url, true);
    xmlhttp.send();
    // no return statement here!
}

this.Construct = function(){
    this.Parse('mind/words.json', function(error, words) {
        // utilize the retrieved words here
    });
}}

Answer 2

Your code contains several errors, with the main one being that it does not account for the asynchronous nature of the XMLHttpRequest function. This means it will not return a value in the same way as synchronous functions. To learn more about this issue, you can refer to this resource: How to make a function wait until a callback has been called using node.js. In essence, you need to use a callback function in your parse method to handle the return value instead of relying on a traditional return statement. If this concept is new to you, I recommend researching "javascript callbacks" to understand it better.

Additionally, there are some minor issues in your code such as returning 'result' from 'Parse' without setting it to anything first, and assigning 'words' in multiple places in a confusing manner. These problems can be resolved once you address the synchronization and asynchrony issues.

EDIT:

The suggested fix involves modifying your code as follows:

this.Parse = function(u, callback){ // introducing the "callback" parameter
    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); // passing the words to the callback function
        }
    }
    xmlhttp.open("GET", url, true);
    xmlhttp.send();
    // no return statement here!
}

this.Construct = function(){
    this.Parse('mind/words.json', function(error, words) {
        // utilize the retrieved words here
    });
}}

Error encountered while attempting to parse JSON: Unable to access property

Answer №1

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