JavaScript 2D Array Uncaught TypeError

Question

Working on a project where I need to add points to an array. Here is the code for creating and then trying to add an object to the array:

var points = [];
    points[0] = [];
    points[0][0] = 0; //x-coord
    points[0][1] = 0; //y-coord

points[points.length][0] = x; //Breaks on this line
points[points.length][1] = y;

The exact error I get is Uncaught TypeError: Cannot set property '0' of undefined. This code is run every time a button is pressed and the other values are already ints that are set. I thought that JavaScript allowed you to set values in arrays as you go?

I'd recommend using an array of objects instead: points = []; points[0] = {x: 0, y: 0}; — jimw
– jimw, Commented May 11, 2012 at 17:38

Alexander Pavlov · Accepted Answer · 2012-05-11 17:30:33Z

2

The last array index is points.length - 1. Now you are addressing an element past the end of array (which is undefined) and trying to assign its "0" property a value of x. Setting properties on undefined is not allowed.

answered May 11, 2012 at 17:30

Alexander Pavlov

32.4k5 gold badges71 silver badges95 bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

wchristiansen Over a year ago

Thanks, as soon as I posted, I realize I was initializing the second array and had to write a function to instantiate the inner arrays. Thanks for the prompt answer though!

Joseph · Accepted Answer · 2012-05-11 17:36:08Z

1

your array contains:

var points = [
    [0,0]
]

However

points[points.length]
//is
points[1]

which does not exist. you must create the array first, before you actually add stuff to it

Here's a sample

var length = points.length;  //cache value, since length is "live"
points[length] = [];         //using the cached length, create new array
points[length][0] = x;       //add values
points[length][1] = y;

edited May 11, 2012 at 17:36

answered May 11, 2012 at 17:31

Joseph

120k31 gold badges186 silver badges238 bronze badges

Collectives™ on Stack Overflow

JavaScript 2D Array Uncaught TypeError

2 Answers 2

1 Comment

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

1 Comment

Comments

Your Answer

Sign up or log in

Post as a guest

Related