Get this error: “Uncaught TypeError: Cannot read property ‘0’ of undefined”

by

EDIT: Just seen youve posted your html in a comment – your html does not have any form elements, hence document.forms[0] will be undefined.

In general, the error “Uncaught TypeError: Cannot read property '0' of undefined” comes about when you try to access a variable whose value is undefined using an array-like square bracket syntax.

The code below demonstrates this in a rather convoluted way (Setting up a function with a parameter, only to call the function without passing an argument, making that parameter undefined. Watch the console for the error) 

function doSomething(myArray){
   console.log(myArray[0]); 
}

doSomething();

Now, in your code this really only boils down to 1 thing, either document.forms is undefined or on one of the iterations of your loop document.forms[0].correctX is undefined (where X is a value from your loop).

Without seeing your HTML, it is impossible to ascertain what exactly is happening, but you can debug it by stepping through until the error is occurred and looking at the value of i.

As an aside, and has been mentioned in comments eval is unnecessary here, you can use the square bracket notation to dynamically access objects, and this may also help you debug:

var isYesChecked = document.forms[0]["correct"+i][0].checked;

More Related Contents:

  1. Why jQuery .load() is not working? [closed]
  2. jQuery+JavaScript: Create custom ajax call function with single object as argument [closed]
  3. How to disable scrolling temporarily?
  4. jqGrid does not render correctly in Chrome/Chrome Frame
  5. How to clone and change id?
  6. jQuery post() with serialize and extra data
  7. Clear Text Selection with JavaScript
  8. Triggering a JavaScript click() event at specific coordinates
  9. Wait till a Function with animations is finished until running another Function
  10. Listening for Youtube Event in JavaScript or jQuery
  11. How to get html elements from an object tag?
  12. jQuery selector doesn’t update after dynamically adding new elements
  13. How do I remove javascript validation from my eclipse project?
  14. How can I detect browser type using jQuery?
  15. get a list of all folders in directory [closed]
  16. Using javascript and jquery, to populate related select boxes with array structure
  17. Is it OK to add your own attributes to HTML elements? [duplicate]
  18. Scale a div to fit in window but preserve aspect ratio
  19. How to Determine if ::before is applied to an element?
  20. Dynamically change CSS of link based on current page
  21. Submit form after calling e.preventDefault()
  22. Age from Date of Birth using JQuery
  23. one document.createElement, append it twice, only shows once
  24. jQuery in Chrome returns “block” instead of “inline”
  25. Change form values after submit button pressed
  26. How to write into a file (in user directory) using JavaScript?
  27. Submit jQuery UI dialog on
  28. Put quotes around a variable string in JavaScript
  29. How to show loading status in percentage for ajax response?
  30. Uncaught NotFoundError: Failed to execute ‘insertBefore’ on ‘Node’: The node before which the new node is to be inserted is not a child of this node

Leave a Comment