Cannot read property of undefined Processing.js - processing.js

Question

I have made an object with the object elevation, yet when a try accessing it using an array, the debugger states it is undefined.
var h = 30;
var disk = function(pos,elv){
this.pos = pos;
this.elv = elv;
rect(56 * pos, 369-pos * h + h, 95, h, 2);
};
var disks = [
{position:1,
elevation:1},
{position:1,
elevation:2},
{position:1,
elevation:3},
{position:1,
elevation:4},
{position:1,
elevation:5},
{position:1,
elevation:6}
];
fill(0, 136, 255);
for(var i = 0; i<= 6; i++){
var diskNum = disks[i];
disk(1,diskNum.elevation);
}
};
I expected the program to draw the rectangles on top of each other, but the program displays Cannot read property elevation of undefined.
https://www.khanacademy.org/computer-programming/new-program/5983920391094272

Answer 1

You have 6 elements in your disks array, at indexes 0, 1, 2, 3, 4, and 5.
Then you use this for loop to iterate over them:
for(var i = 0; i <= 6; i++){
Since you're using the <= operator, this includes 7 total indexes: 0, 1, 2, 3, 4, 5, and 6.
You probably just want the < operator instead.
Also note that it doesn't make a ton of sense to store this.pos and this.elv since you never use them for anything.
When I fix the above error, I see a rectangle. Not quite sure what the program is meant to do, but this at least fixes your error.