How can I create a two dimensional array in JavaScript?

I have been reading online and some places say it isn't possible, some say it is and then give an example and others refute the example, etc.

  1. How do I declare a 2 dimensional array in JavaScript? (assuming it's possible)

  2. How would I access its members? (myArray[0][1] or myArray[0,1]?)

Answers:

Answer

var items = [
  [1, 2],
  [3, 4],
  [5, 6]
];
console.log(items[0][0]); // 1
console.log(items[0][1]); // 2
console.log(items[1][0]); // 3
console.log(items[1][1]); // 4
console.log(items);

Answer

You simply make each item within the array an array.

var x = new Array(10);

for (var i = 0; i < x.length; i++) {
  x[i] = new Array(3);
}

console.log(x);

Answer

Similar to activa's answer, here's a function to create an n-dimensional array:

function createArray(length) {
    var arr = new Array(length || 0),
        i = length;

    if (arguments.length > 1) {
        var args = Array.prototype.slice.call(arguments, 1);
        while(i--) arr[length-1 - i] = createArray.apply(this, args);
    }

    return arr;
}

createArray();     // [] or new Array()

createArray(2);    // new Array(2)

createArray(3, 2); // [new Array(2),
                   //  new Array(2),
                   //  new Array(2)]
Answer

Javascript only has 1-dimensional arrays, but you can build arrays of arrays, as others pointed out.

The following function can be used to construct a 2-d array of fixed dimensions:

function Create2DArray(rows) {
  var arr = [];

  for (var i=0;i<rows;i++) {
     arr[i] = [];
  }

  return arr;
}

The number of columns is not really important, because it is not required to specify the size of an array before using it.

Then you can just call:

var arr = Create2DArray(100);

arr[50][2] = 5;
arr[70][5] = 7454;
// ...
Answer

The easiest way:

var myArray = [[]];
Answer

How to create an empty two dimensional array (one-line)

Array.from(Array(2), () => new Array(4))

2 and 4 being first and second dimensions respectively.

We are making use of Array.from, which can take an array-like param and an optional mapping for each of the elements.

Array.from(arrayLike[, mapFn[, thisArg]])

var arr = Array.from(Array(2), () => new Array(4));
arr[0][0] = 'foo';
console.info(arr);

The same trick can be used to Create a JavaScript array containing 1...N


Alternatively (but more inefficient 12% with n = 10,000)

Array(2).fill(null).map(() => Array(4))

The performance decrease comes with the fact that we have to have the first dimension values initialized to run .map. Remember that Array will not allocate the positions until you order it to through .fill or direct value assignment.

var arr = Array(2).fill(null).map(() => Array(4));
arr[0][0] = 'foo';
console.info(arr);


Follow up

Here's a method that appears correct, but has issues.

 Array(2).fill(Array(4)); // BAD! Rows are copied by reference

While it does return the apparently desired two dimensional array ([ [ <4 empty items> ], [ <4 empty items> ] ]), there a catch: first dimension arrays have been copied by reference. That means a arr[0][0] = 'foo' would actually change two rows instead of one.

var arr = Array(2).fill(Array(4));
arr[0][0] = 'foo';
console.info(arr);
console.info(arr[0][0], arr[1][0]);

Answer

The reason some say that it isn't possible is because a two dimensional array is really just an array of arrays. The other comments here provide perfectly valid methods of creating two dimensional arrays in JavaScript, but the purest point of view would be that you have a one dimensional array of objects, each of those objects would be a one dimensional array consisting of two elements.

So, that's the cause of the conflicting view points.

Answer

Few people show the use of push:
To bring something new, I will show you how to initialize the matrix with some value, example: 0 or an empty string "".
Reminding that if you have a 10 elements array, in javascript the last index will be 9!

function matrix( rows, cols, defaultValue){

  var arr = [];

  // Creates all lines:
  for(var i=0; i < rows; i++){

      // Creates an empty line
      arr.push([]);

      // Adds cols to the empty line:
      arr[i].push( new Array(cols));

      for(var j=0; j < cols; j++){
        // Initializes:
        arr[i][j] = defaultValue;
      }
  }

return arr;
}

usage examples:

x = matrix( 2 , 3,''); // 2 lines, 3 cols filled with empty string
y = matrix( 10, 5, 0);// 10 lines, 5 cols filled with 0
Answer

Two-liner:

var a = []; 
while(a.push([]) < 10);

It will generate an array a of the length 10, filled with arrays. (Push adds an element to an array and returns the new length)

Answer

The sanest answer seems to be

var nrows = ~~(Math.random() * 10);
var ncols = ~~(Math.random() * 10);
console.log(`rows:${nrows}`);
console.log(`cols:${ncols}`);
var matrix = new Array(nrows).fill(0).map(row => new Array(ncols).fill(0));
console.log(matrix);


Note we can't directly fill with the rows since fill uses shallow copy constructor, therefore all rows would share the same memory...here is example which demonstrates how each row would be shared (taken from other answers):

// DON'T do this: each row in arr, is shared
var arr = Array(2).fill(Array(4));
arr[0][0] = 'foo'; // also modifies arr[1][0]
console.info(arr);
Answer

The easiest way:

var arr  = [];

var arr1 = ['00','01'];
var arr2 = ['10','11'];
var arr3 = ['20','21'];

arr.push(arr1);
arr.push(arr2);
arr.push(arr3);

alert(arr[0][1]); // '01'
alert(arr[1][1]); // '11'
alert(arr[2][0]); // '20'
Answer

This is what i achieved :

var appVar = [[]];
appVar[0][4] = "bineesh";
appVar[0][5] = "kumar";
console.log(appVar[0][4] + appVar[0][5]);
console.log(appVar);

This spelled me bineeshkumar

Answer

Two dimensional arrays are created the same way single dimensional arrays are. And you access them like array[0][1].

var arr = [1, 2, [3, 4], 5];

alert (arr[2][1]); //alerts "4"
Answer

I'm not sure if anyone has answered this but I found this worked for me pretty well -

var array = [[,],[,]]

eg:

var a = [[1,2],[3,4]]

For a 2 dimensional array, for instance.

Answer

To create a 2D array in javaScript we can create an Array first and then add Arrays as it's elements. This method will return a 2D array with the given number of rows and columns.

function Create2DArray(rows,columns) {
   var x = new Array(rows);
   for (var i = 0; i < rows; i++) {
       x[i] = new Array(columns);
   }
   return x;
}

to create an Array use this method as below.

var array = Create2DArray(10,20);
Answer

To create a non-sparse "2D" array (x,y) with all indices addressable and values set to null:

let 2Darray = new Array(x).fill(null).map(item =>(new Array(y).fill(null))) 

bonus "3D" Array (x,y,z)

let 3Darray = new Array(x).fill(null).map(item=>(new Array(y).fill(null)).map(item=>Array(z).fill(null)))

Variations and corrections on this have been mentioned in comments and at various points in response to this question but not as an actual answer so I am adding it here.

It should be noted that (similar to most other answers) this has O(x*y) time complexity so it probably not suitable for very large arrays.

Answer

Use Array Comprehensions

In JavaScript 1.7 and higher you can use array comprehensions to create two dimensional arrays. You can also filter and/or manipulate the entries while filling the array and don't have to use loops.

var rows = [1, 2, 3];
var cols = ["a", "b", "c", "d"];

var grid = [ for (r of rows) [ for (c of cols) r+c ] ];

/* 
         grid = [
            ["1a","1b","1c","1d"],
            ["2a","2b","2c","2d"],
            ["3a","3b","3c","3d"]
         ]
*/

You can create any n x m array you want and fill it with a default value by calling

var default = 0;  // your 2d array will be filled with this value
var n_dim = 2;
var m_dim = 7; 

var arr = [ for (n of Array(n_dim)) [ for (m of Array(m_dim) default ]] 
/* 
         arr = [
            [0, 0, 0, 0, 0, 0, 0],
            [0, 0, 0, 0, 0, 0, 0],
         ]
*/

More examples and documentation can be found here.

Please note that this is not a standard feature yet.

Answer

For one liner lovers Array.from()

// creates 8x8 array filed with "0"    
const arr2d = Array.from({ length: 8 }, () => Array.from({ length: 8 }, () => "0"))

Another one (from comment by dmitry_romanov) use Array().fill()

// creates 8x8 array filed with "0"    
const arr2d = Array(8).fill(0).map(() => Array(8).fill("0"))

Using ES6+ spread operator ("inspired" by InspiredJW answer :) )

// same as above just a little shorter
const arr2d = [...Array(8)].map(() => Array(8).fill("0"))
Answer

My approach is very similar to @Bineesh answer but with a more general approach.

You can declare the double array as follows:

var myDoubleArray = [[]];

And the storing and accessing the contents in the following manner:

var testArray1 = [9,8]
var testArray2 = [3,5,7,9,10]
var testArray3 = {"test":123}
var index = 0;

myDoubleArray[index++] = testArray1;
myDoubleArray[index++] = testArray2;
myDoubleArray[index++] = testArray3;

console.log(myDoubleArray[0],myDoubleArray[1][3], myDoubleArray[2]['test'],) 

This will print the expected output

[ 9, 8 ] 9 123
Answer

I found below is the simplest way:

var array1 = [[]];   
array1[0][100] = 5; 

alert(array1[0][100]);
alert(array1.length);
alert(array1[0].length);
Answer

Javascript does not support two dimensional arrays, instead we store an array inside another array and fetch the data from that array depending on what position of that array you want to access. Remember array numeration starts at ZERO.

Code Example:

/* Two dimensional array that's 5 x 5 

       C0 C1 C2 C3 C4 
    R0[1][1][1][1][1] 
    R1[1][1][1][1][1] 
    R2[1][1][1][1][1] 
    R3[1][1][1][1][1] 
    R4[1][1][1][1][1] 
*/

var row0 = [1,1,1,1,1],
    row1 = [1,1,1,1,1],
    row2 = [1,1,1,1,1],
    row3 = [1,1,1,1,1],
    row4 = [1,1,1,1,1];

var table = [row0,row1,row2,row3,row4];
console.log(table[0][0]); // Get the first item in the array
Answer

I had to make a flexible array function to add "records" to it as i needed and to be able to update them and do whatever calculations e needed before i sent it to a database for further processing. Here's the code, hope it helps :).

function Add2List(clmn1, clmn2, clmn3) {
    aColumns.push(clmn1,clmn2,clmn3); // Creates array with "record"
    aLine.splice(aPos, 0,aColumns);  // Inserts new "record" at position aPos in main array
    aColumns = [];    // Resets temporary array
    aPos++ // Increments position not to overlap previous "records"
}

Feel free to optimize and / or point out any bugs :)

Answer

Here's a quick way I've found to make a two dimensional array.

function createArray(x, y) {
    return Array.apply(null, Array(x)).map(e => Array(y));
}

You can easily turn this function into an ES5 function as well.

function createArray(x, y) {
    return Array.apply(null, Array(x)).map(function(e) {
        return Array(y);
    });
}

Why this works: the new Array(n) constructor creates an object with a prototype of Array.prototype and then assigns the object's length, resulting in an unpopulated array. Due to its lack of actual members we can't run the Array.prototype.map function on it.

However, when you provide more than one argument to the constructor, such as when you do Array(1, 2, 3, 4), the constructor will use the arguments object to instantiate and populate an Array object correctly.

For this reason, we can use Array.apply(null, Array(x)), because the apply function will spread the arguments into the constructor. For clarification, doing Array.apply(null, Array(3)) is equivalent to doing Array(null, null, null).

Now that we've created an actual populated array, all we need to do is call map and create the second layer (y).

Answer

Below one, creates a 5x5 matrix and fill them with null

var md = [];
for(var i=0; i<5; i++) {
    md.push(new Array(5).fill(null));
}

console.log(md);

Answer

var playList = [
  ['I Did It My Way', 'Frank Sinatra'],
  ['Respect', 'Aretha Franklin'],
  ['Imagine', 'John Lennon'],
  ['Born to Run', 'Bruce Springsteen'],
  ['Louie Louie', 'The Kingsmen'],
  ['Maybellene', 'Chuck Berry']
];

function print(message) {
  document.write(message);
}

function printSongs( songs ) {
  var listHTML;
  listHTML = '<ol>';
  for ( var i = 0; i < songs.length; i += 1) {
    listHTML += '<li>' + songs[i][0] + ' by ' + songs[i][1] + '</li>';
  }
  listHTML += '</ol>';
  print(listHTML);
}

printSongs(playList);

Answer

There is another solution, that does not force you to pre-define the size of the 2d array, and that is very concise.

var table = {}
table[[1,2]] = 3 // Notice the double [[ and ]]
console.log(table[[1,2]]) // -> 3

This works because, [1,2] is transformed into a string, that is used as a string key for the table object.

Answer

Row and Column sizes of an array known only at the run time then following method could be used for creating a dynamic 2d array.

var num = '123456';
var row = 3; // Known at run time
var col = 2; // Known at run time
var i = 0;

var array2D = [[]];
for(var r = 0; r < row; ++r)
{
    array2D[r] = [];
    for(var c = 0; c < col; ++c)
    {
        array2D[r][c] = num[i++];
    }
}
console.log(array2D); 
// [[ '1', '2' ], 
//  [ '3', '4' ], 
//  [ '5', '6' ]]

console.log(array2D[2][1]); // 6
Answer

ES6+, ES2015+ can do this in even simpler way


Creating 3 x 2 Array filled with true

[...Array(3)].map(item => Array(2).fill(true))
Answer

You could allocate an array of rows, where each row is an array of the same length. Or you could allocate a one-dimensional array with rows*columns elements and define methods to map row/column coordinates to element indices.

Whichever implementation you pick, if you wrap it in an object you can define the accessor methods in a prototype to make the API easy to use.

Answer

I found that this code works for me:

var map = [
    []
];

mapWidth = 50;
mapHeight = 50;
fillEmptyMap(map, mapWidth, mapHeight);

...

function fillEmptyMap(array, width, height) {
    for (var x = 0; x < width; x++) {
        array[x] = [];
        for (var y = 0; y < height; y++) {

            array[x][y] = [0];
        }
    }
}
Answer

Performance

Today 2020.02.05 I perform tests on MacOs HighSierra 10.13.6 on Chrome v79.0, Safari v13.0.4 and Firefox v72.0, for chosen solutions.

Conclusions for non-initialised 2d array

  • esoteric solution {}/arr[[i,j]] (N) is fastest for big and small arrays and it looks like it is good choice for big sparse arrays
  • solutions based on for-[]/while (A,G) are fast and they are good choice for small arrays.
  • solutions for-[] (B,C) are fast and they are good choice for big arrays
  • solutions based on Array..map/from/fill (I,J,K,L,M) are quite slow for small arrays, and quite fast for big arrays
  • surprinsingly for-Array(n) (B,C) is much slower on safari than for-[] (A)
  • surprinsingly for-[] (A) for big array is slow on all browsers
  • solutions K is slow for small arrays for all browsers
  • solutions A,E,G are slow for big arrays for all browsers
  • solution M is slowest for all arrays on all browsers

enter image description here

Conclusions for initialised 2d array

  • solutions based on for/while (A,B,C,D,E,G) are fastest/quite fast for small arrays on all browsers
  • solutions based on for (A,B,C,E) are fastest/quite fast for big arrays on all browsers
  • solutions based on Array..map/from/fill (I,J,K,L,M) are medium fast or slow for small arrays on all browsers
  • solutions F,G,H,I,J,K,L for big arrays are medium or fast on chrome and safari but slowest on firefox.
  • esoteric solution {}/arr[[i,j]] (N) is slowest for small and big arrays on all browsers

enter image description here

Details

Test for solutions which not fill (initialise) output array

We test speed of solutions for

  • small arrays (12 elements) - you can perform tests on your machine HERE
  • big arrays (1 million elements) arrays - you can perform tests on your machine HERE

function A(r) {
  var arr = [];
  for (var i = 0; i < r; i++) arr[i] = [];
  return arr;
}

function B(r, c) {
  var arr = new Array(r);
  for (var i = 0; i < arr.length; i++) arr[i] = new Array(c);
  return arr;
}

function C(r, c) {
  var arr = Array(r);
  for (var i = 0; i < arr.length; i++) arr[i] = Array(c);
  return arr;
}

function D(r, c) {
  // strange, but works
  var arr = [];
  for (var i = 0; i < r; i++) {
    arr.push([]);
    arr[i].push(Array(c));
  }
  return arr;
}

function E(r, c) {
  let array = [[]];
  for (var x = 0; x < c; x++) {
    array[x] = [];
    for (var y = 0; y < r; y++) array[x][y] = [0];
  }
  return array;
}

function F(r, c) {
  var makeArray = function(dims, arr) {
    if (dims[1] === undefined) {
      return Array(dims[0]);
    }

    arr = Array(dims[0]);

    for (var i = 0; i < dims[0]; i++) {
      arr[i] = Array(dims[1]);
      arr[i] = makeArray(dims.slice(1), arr[i]);
    }

    return arr;
  }
  return makeArray([r, c]);
}

function G(r) {
  var a = [];
  while (a.push([]) < r);
  return a;
}

function H(r,c) {
  function createArray(length) {
    var arr = new Array(length || 0),
        i = length;

    if (arguments.length > 1) {
        var args = Array.prototype.slice.call(arguments, 1);
        while(i--) arr[length-1 - i] = createArray.apply(this, args);
    }

    return arr;
  }
  return createArray(r,c);
}

function I(r, c) {
  return [...Array(r)].map(x => Array(c));
}

function J(r, c) {
  return Array(r).fill(0).map(() => Array(c));
}

function K(r, c) {
  return Array.from(Array(r), () => Array(c));
}

function L(r, c) {
  return Array.from({length: r}).map(e => Array(c));
}

function M(r, c) {
  return Array.from({length: r}, () => Array.from({length: c}, () => {}));
}

function N(r, c) {
  return {}
}



// -----------------------------------------------
// SHOW
// -----------------------------------------------

log = (t, f) => {
  let A = f(3, 4); // create array with 3 rows and 4 columns
  A[1][2] = 6 // 2-nd row 3nd column set to 6
  console.log(`${t}[1][2]: ${A[1][2]}, full: ${JSON.stringify(A).replace(/null/g,'x')}`);
}

log2 = (t, f) => {
  let A = f(3, 4); // create array with 3 rows and 4 columns
  A[[1,2]] = 6 // 2-nd row 3nd column set to 6
  console.log(`${t}[1][2]: ${A[[1,2]]}, full: ${JSON.stringify(A).replace(/null/g,'x')}`);
}

log('A', A);
log('B', B);
log('C', C);
log('D', D);
log('E', E);
log('F', F);
log('G', G);
log('H', H);
log('I', I);
log('J', J);
log('K', K);
log('L', L);
log('M', M);
log2('N', N);
This is presentation of solutions - not benchmark

Test for solutions which fill (initialise) output array

We test speed of solutions for

  • small arrays (12 elements) - you can perform tests on your machine HERE
  • big arrays (1 million elements) arrays - you can perform tests on your machine HERE

function A(r, c, def) {
  var arr = [];
  for (var i = 0; i < r; i++) arr[i] = Array(c).fill(def);
  return arr;
}

function B(r, c, def) {
  var arr = new Array(r);
  for (var i = 0; i < arr.length; i++) arr[i] = new Array(c).fill(def);
  return arr;
}

function C(r, c, def) {
  var arr = Array(r);
  for (var i = 0; i < arr.length; i++) arr[i] = Array(c).fill(def);
  return arr;
}

function D(r, c, def) {
  // strange, but works
  var arr = [];
  for (var i = 0; i < r; i++) {
    arr.push([]);
    arr[i].push(Array(c));
  }
  for (var i = 0; i < r; i++) for (var j = 0; j < c; j++) arr[i][j]=def
  return arr;
}

function E(r, c, def) {
  let array = [[]];
  for (var x = 0; x < c; x++) {
    array[x] = [];
    for (var y = 0; y < r; y++) array[x][y] = def;
  }
  return array;
}

function F(r, c, def) {
  var makeArray = function(dims, arr) {
    if (dims[1] === undefined) {
      return Array(dims[0]).fill(def);
    }

    arr = Array(dims[0]);

    for (var i = 0; i < dims[0]; i++) {
      arr[i] = Array(dims[1]);
      arr[i] = makeArray(dims.slice(1), arr[i]);
    }

    return arr;
  }
  return makeArray([r, c]);
}

function G(r, c, def) {
  var a = [];
  while (a.push(Array(c).fill(def)) < r);
  return a;
}

function H(r,c, def) {
  function createArray(length) {
    var arr = new Array(length || 0),
        i = length;

    if (arguments.length > 1) {
        var args = Array.prototype.slice.call(arguments, 1);
        while(i--) arr[length-1 - i] = createArray.apply(this, args).fill(def);
    }

    return arr;
  }
  return createArray(r,c);
}

function I(r, c, def) {
  return [...Array(r)].map(x => Array(c).fill(def));
}

function J(r, c, def) {
  return Array(r).fill(0).map(() => Array(c).fill(def));
}

function K(r, c, def) {
  return Array.from(Array(r), () => Array(c).fill(def));
}

function L(r, c, def) {
  return Array.from({length: r}).map(e => Array(c).fill(def));
}

function M(r, c, def) {
  return Array.from({length: r}, () => Array.from({length: c}, () => def));
}

function N(r, c, def) {
  let arr={};
  for (var i = 0; i < r; i++) for (var j = 0; j < c; j++) arr[[i,j]]=def;
  return arr;
}



// -----------------------------------------------
// SHOW
// -----------------------------------------------

log = (t, f) => {
  let A = f(1000,1000,7); // create array with 1000 rows and 1000 columns, 
                          // each array cell initilised by 7
  A[800][900] = 5         // 800nd row and 901nd column set to 5
  console.log(`${t}[1][2]: ${A[1][2]}, ${t}[800][901]: ${A[800][900]}`);
}

log2 = (t, f) => {
  let A = f(1000,1000,7); // create array with 1000 rows and 1000 columns, 
                          // each array cell initilised by 7
  A[[800,900]] = 5            // 800nd row 900nd column set to 5
  console.log(`${t}[1][2]: ${A[[1,2]]}, ${t}[800][900]: ${A[[800,900]]}`);
}

log('A', A);
log('B', B);
log('C', C);
log('D', D);
log('E', E);
log('F', F);
log('G', G);
log('H', H);
log('I', I);
log('J', J);
log('K', K);
log('L', L);
log('M', M);
log2('N', N);
This is presentation of solutions - not benchmark

enter image description here

Answer

To create an 4x6 array, simply do this

const x = [...Array(6)].map(elem => new Array(4))
Answer

One liner to create a m*n 2 dimensional array filled with 0.

new Array(m).fill(new Array(n).fill(0));

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