How to create empty 2d array in javascript?

Question

How do I create an empty 2D array in Javascript (without knowing how many rows or columns there will be in the new array)?

If it\'s a simple array var newArra

Answer 1

Two things:

1) The array length property improperly reports the array length if called after the var myArray = [[],[]]; statement. Technically, since the empty arrays are defined, they are getting counted by the length property, but in the spirit of the length property it really should return 0, because no non-empty elements have been added to any of the arrays.

A minimum work around is to use two nested for( in ) loops, one for the 1st array and one for the 2nd array, and to count the non-undefined elements.

2) Extending Siamak A.Motlagh example and adding a arr([2][4]) = 'Hi Mr.C'; assignment fails with an "Uncaught TypeError: Cannot set property '4' of undefined" error.

See the jsFiddle: http://jsfiddle.net/howardb1/zq8oL2ds/

Here is a copy of that code:

var arr = [[],[]];

alert( arr.length );  // wrong!

var c = 0;
for( var i in arr )
  for( var j in arr[ i ] )
    if( arr[ i ][ j ] != undefined )
      ++c;
alert( c );  // correct

arr[0][2] = 'Hi Mr.A';
alert(arr[0][2]);

arr[1][3] = 'Hi Mr.B';
alert(arr[1][3]);

arr[2][4] = 'Hi Mr.C';  // At this point I'm getting VM558:62 Uncaught TypeError: Cannot set property '4' of undefined
alert(arr[2][4]);

var c = 0;
for( var i in arr )
  for( var j in arr[ i ] )
    if( arr[ i ][ j ] != undefined )
      ++c;
alert( c );

Why does the third assignment fail? What about the [[],[]] creation statement told it that the first array was valid for 0 and 1, but not 2 or that 2 and 3 were ok for the second array, but not 4?

Most importantly, how would I define an Array in an Array that could hold date objects in the first and second arrays. I'm using the jQuery-UI DatePicker, which expects an array of dates, as in date objects, which I've extended to use a second date array to contain date objects that contain times so I can keep track of multiple dates, and multiple times per day.

Thanks.

Answer 2

var myArray = [
    ["cats","dogs","monkeys","horses"],
    ["apples","oranges","pears","bananas"]
];
document.write(myArray[0][2]) //returns "monkeys"

Answer 3

This also works as an expression:

var twoDarr= new Array(desiredLength);
for (i=0;i<twoDarr.length;i++) {twoDarr[i]=[];} 

I don't know how it pars in terms of performance with the rest of the answers here, if you have a clue let me know in the comments.

If you don't know the length of the array beforehand pls have in mind that you can use either push([]), or splice() if you want to push/remove/replace a new element in place of an existing one.

Answer 4

const grid = new Array(n).fill(new Array(n))

Answer 5

There are no two dimensional arrays in Javascript.

To accomplish the effect of a two dimensional array, you use an array of arrays, also known as a jagged array (because the inner arrays can have different length).

An empty jagged array is created just like any other empty array:

var myArray = new Array();

You can also use an empty array literal:

var myArray = [];

To put any items in the jagged array, you first have to put inner arrays in it, for example like this:

myArray.push([]);
myArray[0][0] = 'hello';

You can also create an array that contains a number of empty arrays from start:

var myArray = [[],[],[]];

That gives you a jagged array without any items, but which is prepared with three inner arrays.

As it's an array of arrays, you access the items using myArray[0][1].

Answer 6

Yes you can create an empty array and then push data into it. There is no need to define the length first in JavaScript.
Check out jsFiddle Live Demo

Define:

var arr = [[],[]];

Push data:

arr[0][2] = 'Hi Mr.A';
arr[1][3] = 'Hi Mr.B';

Read data:

alert(arr[0][2]);
alert(arr[1][3]);

Update

Here is also a video recommended by Brady Dowling:
Create a 2D array