javascript problem with class method for loading json file

Question

javascript problem with class method for loading json file

I've encountered an issue with my class setup below. Despite most things working, the console keeps throwing an error when json.onload is triggered. The error message reads "

Uncaught TypeError: Cannot read property of 'push' of undefined

".

How can I resolve this error?

var Clazz = new function(){
  this.library = [];

  this.add_library = function(file){
    var json = new XMLHttpRequest();
    json.overrideMimeType("application/json");
    json.open("GET", file, false); //needs to be synchronise
    json.onload = function(){
      this.library.push(JSON.parse(json.responseText));
      do something...
    };
    json.send(null);
  };
}

Clazz.add_library("path/file.json");

Solution

Simply change this.library.push(...); to Clazz.library.push(...);

javascript json

Answer 1

Answer №1

The reason for this behavior is the change in scope within the function this.add_library

var Class = new function(){
  this.library = [];
    var self = this;

  this.add_library = function(file){
    var json = new XMLHttpRequest();
    json.overrideMimeType("application/json");
    json.open("GET", file, false); //needs to be synchronise

    json.onload = function(){
        console.log(self)
      self.library.push(JSON.parse(json.responseText));
      //do something...
    };
    json.send(null);
  };
}

new Class.add_library("path/file.json");

Answer 2

The reason for this behavior is the change in scope within the function this.add_library

var Class = new function(){
  this.library = [];
    var self = this;

  this.add_library = function(file){
    var json = new XMLHttpRequest();
    json.overrideMimeType("application/json");
    json.open("GET", file, false); //needs to be synchronise

    json.onload = function(){
        console.log(self)
      self.library.push(JSON.parse(json.responseText));
      //do something...
    };
    json.send(null);
  };
}

new Class.add_library("path/file.json");

Answer 3

Answer №2

Experiment with this code snippet:

var MyClass = new function(){
  this.collection = [];

  this.add_item = (item) => {
    var data = new XMLHttpRequest();
    data.overrideMimeType("application/json");
    data.open("GET", item, false); //must be synchronized
    data.onload = () => {
      this.collection.push(JSON.parse(data.responseText));
      // do something...
    };
    data.send(null);
  };
}

Alternatively, you could try using the .bind(this) method to address the issue with the visibility of this within the function's scope.

Answer 4

Experiment with this code snippet:

var MyClass = new function(){
  this.collection = [];

  this.add_item = (item) => {
    var data = new XMLHttpRequest();
    data.overrideMimeType("application/json");
    data.open("GET", item, false); //must be synchronized
    data.onload = () => {
      this.collection.push(JSON.parse(data.responseText));
      // do something...
    };
    data.send(null);
  };
}

Alternatively, you could try using the .bind(this) method to address the issue with the visibility of this within the function's scope.

Answer 5

Answer №3

The issue at hand revolves around the concept of context in JavaScript. JavaScript has different scenarios for the this keyword, and your example involves two of them:

Method invocation - this refers to the object calling the method.
Function invocation - this points to the global object or undefined in strict mode.

Let's take a closer look at your code (I will make some adjustments for clarity):

var Clazz = {
    library: [], 

    add_library: function(file){ 
        // When using `this.library` here, it refers to the `library` property of the Clazz object
        // and it works fine since `this` points to the Clazz object
        json.onload = function(){
            this.library.push(JSON.parse(json.responseText));
            // However, in this section `this` refers to the global object
            // because `json.onload` is a function, even though it is within the `add_library` method
        };
    };
}

So, how can we solve this issue? Here are a few suggestions:

Utilize .bind():

json.onload = function(){
    this.library.push(JSON.parse(json.responseText));
}.bind(Clazz);

Assign the context to a variable:

const self = this;
json.onload = function(){
    self.library.push(JSON.parse(json.responseText));
}

Use arrow function:

json.onload = () => {
    this.library.push(JSON.parse(json.responseText));
}

Answer 6

The issue at hand revolves around the concept of context in JavaScript. JavaScript has different scenarios for the this keyword, and your example involves two of them:

Method invocation - this refers to the object calling the method.
Function invocation - this points to the global object or undefined in strict mode.

Let's take a closer look at your code (I will make some adjustments for clarity):

var Clazz = {
    library: [], 

    add_library: function(file){ 
        // When using `this.library` here, it refers to the `library` property of the Clazz object
        // and it works fine since `this` points to the Clazz object
        json.onload = function(){
            this.library.push(JSON.parse(json.responseText));
            // However, in this section `this` refers to the global object
            // because `json.onload` is a function, even though it is within the `add_library` method
        };
    };
}

So, how can we solve this issue? Here are a few suggestions:

Utilize .bind():

json.onload = function(){
    this.library.push(JSON.parse(json.responseText));
}.bind(Clazz);

Assign the context to a variable:

const self = this;
json.onload = function(){
    self.library.push(JSON.parse(json.responseText));
}

Use arrow function:

json.onload = () => {
    this.library.push(JSON.parse(json.responseText));
}

javascript problem with class method for loading json file

Answer №1

Answer №2

Answer №3

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