Referencing Javascript array element via loop variable

Question

I've come across writing a piece of code where I wanted to reference the 2D array d2_arr[][] in a loop like so.

for (var i=0; i<d2_arr[i].length; i++) {
 //do something
}

Google Script compiler throws an error "cannot read length property from undefined". When I changed [i] for [1], it worked just fine. Could anyone please explain why this is wrong? And a related question: Can a 2D array have a different number of elements in a row? theoretically. An example would be:

[[1,2],[3,4,5],[6,7,8,9],[10,11]]

EDIT. Full code part.

var ids = []; 
 var j = 0; 
 for (var i=0; i<d2_arr[i].length; i++){
 if (d2_arr[i][2]<=0.05){
 ids[j]=d2_arr[i][0];
 j++;
 }
 }

I understood the mistake. Thank you!

• Yaw Boakye: I find the code sample very inadequate. If you want a full diagnosis of why you're getting the cannot read length property from undefined do you mind pasting the whole code (especially d2_arr)? Is it generated dynamically?
• AVX: Yaw, I think I have mixed up things together. I posted the snippet of code to make it clearly for myself as well. From the answer I understood that I took the wrong length. This is my original code. In the EDIT I see that when I go through i as a row, I constrained it by the number of a row's length, which is wrong. Maybe there at some point the compiler threw an arrow due to an error in the array dimensions.

Answer 1

You typically need a nested for loop to traverse a 2-D array

 var d2_arr = [[1,2],[3,4,5],[6,7,8,9],[10,11]]
 
 for (var i=0; i<d2_arr.length; i++){
 for (var j=0; j<d2_arr[i].length; j++){
 console.log(d2_arr[i][j] + ' ')
 }
 }

It is perfectly fine for arrays to be "jagged" and contain uneven sized arrays in the main array.

Here is a fiddle http://jsfiddle.net/7Lr4542s/

Answer 2

Arrays in JS can be of any size and any type. You can combine number and strings in array.

var twoDArray = [[1], ["one", "two"], ["i", "ii", "iii"]];
 for(var i = 0; i < twoDArray.length; i++) {
 for(var j = 0; j < twoDArray[i].length; j++) {
 print(twoDArray[i][j]);
 }
 }
 var threeDArray = [[["1.1.1", "1.1.2"], ["1.2.1", "1.2.2"]], [["1.2.1", "1.2.2"], ["1.2.1", "1.2.2"]], [["2.1.1", "2.1.2"], ["2.2.1", "2.2.2"]], [["2.2.1", "2.2.2"], ["2.2.1", "2.2.2"]]];
 for(var i = 0; i < threeDArray.length; i++) {
 for(var j = 0; j < threeDArray[i].length; j++) {
 for(var k = 0; k < threeDArray[i][j].length; k++) {
 print(twoDArray[i][j][k]);
 }
 }
 }

• AVX: Thank you Naresh! I cannot vote for accepting two answers at the same time unfortunately, but your example of 3D array is something I was actually thinking if that's possible too.