JavaScript Elegance: Error Encountered - Unable to Locate Property 'index of' of an Undefined Value

Question

JavaScript Elegance: Error Encountered - Unable to Locate Property 'index of' of an Undefined Value

As I delve into learning JavaScript through Eloquent Javascript, I encountered an error in one of the chapters that has me stumped. The error message "Cannot read property 'indexOf' of undefined" is appearing with this line of code:
```
return journal.events.indexOf(event) != -1
```

Furthermore, I am puzzled about how that line of code operates. Isn't indexOf supposed to return the first position of occurrence of a specified value (in this case, event)? However, the book shows that

return journal.events.indexOf(event) != -1;

results in either true or false.

var journal = [];

function addEntry(events, didITurnIntoASquirrel) {
    journal.push({
        events: events,
        squirrel: didITurnIntoASquirrel
    });
}

addEntry(["work", "touched tree", "pizza", "running",
          "television"], false);
addEntry(["work", "ice cream", "cauliflower", "lasagna",
          "touched tree", "brushed teeth"], false);
addEntry(["weekend", "cycling", "break", "peanuts",
          "beer"], true); 

function hasEvent(event, entry) {
    return entry.events.indexOf(event) != -1;
}

console.log(hasEvent("pizza", journal));

javascript

Answer 1

Answer №1

The code snippet shows that in the variable journal, an array is being initialized

  var journal = []; <--- Array

Thus, to access the events array within journal, you need to use an index like so:

journal[0].events.indexOf(event)

        ^
        |
        |

Make sure to locate the correct index to retrieve your events efficiently

Answer 2

The code snippet shows that in the variable journal, an array is being initialized

  var journal = []; <--- Array

Thus, to access the events array within journal, you need to use an index like so:

journal[0].events.indexOf(event)

        ^
        |
        |

Make sure to locate the correct index to retrieve your events efficiently

Answer 3

Answer №2

If you're new to learning javascript, please bear with me as I explain some foundational concepts.

Let's start by initializing an empty array:

var journal = [];
console.log(journal); //[]  
//This creates an empty Array named "journal."

When you use the push method on an array, you add an element to the end of it. Learn more about Array.push here.

In this example, we are adding an object to the "journal" array using the push method. Here is how you can create and add an object in a clearer way:

//Create an object first, then add it to the "journal" array.
function addEntry(events, didITurnIntoASquirrel) {
   var temporaryObject = {
      events: events,
      squirrel: didITurnIntoASquirrel
   };
   journal.push(temporaryObject);
}

Now that we have added an object to the "journal" array, let's understand how to access its elements:

console.log(journal); // [temporaryObject]
console.log(journal[0]); - //temporaryObject

Remember, accessing elements inside an array requires traversing through each level of depth step by step.

To check if a specific event exists in the events array, you can define a function like this:

function hasEvent(event, journalEntry) {
    var index = journalEntry.events.indexOf(event);
    var result = (index != -1); 
    return result; 
}

console.log(hasEvent("pizza", journal[0]));
console.log(hasEvent("pizza", journal[1]));
console.log(hasEvent("pizza", journal[2]));

Understanding these fundamental concepts will help you handle data hierarchy effectively in Object Oriented programming scenarios. Feel free to explore the working code on JSFiddle for a clearer visualization.

Answer 4

If you're new to learning javascript, please bear with me as I explain some foundational concepts.

Let's start by initializing an empty array:

var journal = [];
console.log(journal); //[]  
//This creates an empty Array named "journal."

When you use the push method on an array, you add an element to the end of it. Learn more about Array.push here.

In this example, we are adding an object to the "journal" array using the push method. Here is how you can create and add an object in a clearer way:

//Create an object first, then add it to the "journal" array.
function addEntry(events, didITurnIntoASquirrel) {
   var temporaryObject = {
      events: events,
      squirrel: didITurnIntoASquirrel
   };
   journal.push(temporaryObject);
}

Now that we have added an object to the "journal" array, let's understand how to access its elements:

console.log(journal); // [temporaryObject]
console.log(journal[0]); - //temporaryObject

Remember, accessing elements inside an array requires traversing through each level of depth step by step.

To check if a specific event exists in the events array, you can define a function like this:

function hasEvent(event, journalEntry) {
    var index = journalEntry.events.indexOf(event);
    var result = (index != -1); 
    return result; 
}

console.log(hasEvent("pizza", journal[0]));
console.log(hasEvent("pizza", journal[1]));
console.log(hasEvent("pizza", journal[2]));

Understanding these fundamental concepts will help you handle data hierarchy effectively in Object Oriented programming scenarios. Feel free to explore the working code on JSFiddle for a clearer visualization.

Answer 5

Answer №3

Regarding your second inquiry:

"Is IndexOf supposed to provide the initial position of the specified value?"

Absolutely, indeed, indexOf will give back -1 if the desired value is not present in the array.

Therefore, when the event is identified, the statement indexOf(event) != -1 will yield a result of true.

Answer 6

Regarding your second inquiry:

"Is IndexOf supposed to provide the initial position of the specified value?"

Absolutely, indeed, indexOf will give back -1 if the desired value is not present in the array.

Therefore, when the event is identified, the statement indexOf(event) != -1 will yield a result of true.

JavaScript Elegance: Error Encountered - Unable to Locate Property 'index of' of an Undefined Value

Answer №1

Answer №2

Answer №3

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