2D Arrays in Javascript

answer for (a)

var arr = [];
var value = 0;
for(var i = 0; i < 8; i++){
  var tempArr = [];
  for(var j = 0; j < 8; j++){
    tempArr.push(++value);
  }
  arr.push(tempArr);
}

//`arr` has values initialized with 1 - 64

answer for (b)

   var table = "<table>";

   for(var i = 0; i < 8; i++){
      table += "<tr>";
      for(var j = 0; j < 8; j++){
        table += "<td>" + arr[i][j] + "</td>";
      }
      table += "</tr>";
    }
    table += "</table>";

//assume you have an element with id="tbl" in DOM
document.getElementById("tbl").innerHtml = table;

answer for (c)

 var sum = 0 , avg = 0;
   for(var i = 0; i < 8; i++){          
      for(var j = 0; j < 8; j++){
        sum += arr[i][j];
      }
    }
  avg = sum / 64;

 alert("sum: " + sum + " Average: " + avg);

After you create array you can initialize each of the element with another array.

var myarray=new Array(8)

for (i=0; i <8; i++)
    myarray[i]=new Array(8)

Hope this helps.

Cheers !

I made a JSFiddle for this, with comments.

HTML

<button onclick="showCalculations();">Show Calculations!</button>
<div id="container"></div>

JavaScript

//Create the 2-d array
var rows = 8; //Number of rows
var columns = 8; //Number of columns
var list = []; //Array where we will put the data
var index = 0; //Keeps track of which index we're at

for(var i = 0; i < rows; i++){
  var rowList = []; //Create sub-array
  list.push(rowList); //Add the sub-array to our top-level array
  for(var y = 0; y < columns; y++){
    index++; //Increment the index
    rowList.push(index); //Add the index to the current sub-array
  }
}

//Create the Table as HTML string
var table = "<table>"; //Start HTML for table
for(var i = 0; i < list.length; i++){
  var row = list[i];
  table += "<tr>"; //Start HTML for row
  for(var y = 0; y < row.length; y++){
    var column = row[y];
    table += "<td>" + column + "</td>"; //Table cell with content
  }
  table += "</tr>"; //End HTML for row
}
table += "</table>"; //End HTML for table

document.getElementById("container").innerHTML = table; //Find element with id container and set HTML to table

//Calculations
var sum = 0; //Keeps track of the sum
var numberOfNumbers = 0; //Keeps track of how many numbers has been summed

for(var i = 0; i < list.length; i++){
  var row = list[i];
  for(var y = 0; y < row.length; y++){
    var column = row[y];
    sum += column; //Add to sum
    numberOfNumbers++; //Increment amount of numbers
  }
}

var avarage = sum / numberOfNumbers; //Calculate avarage

//Function to alert the values
function showCalculations(){
  alert("Sum: " + sum + "\nAvarage: " + avarage)
}

Note that the creation of the Table and the calculations can be done in the same for-loop. (Acctually all can be, but then we're not making use of the Array we create).

I correct your code. Html:

<button onclick='run()'>Show</button>
<div id='list_'></div>

And js:

function run() {
  x = matrix( 8 , 8, 0 ); 
  alert(x);
  displayInDiv(x);
}
function matrix( rows, cols, defaultValue){
    var arr = [];
    var number = 1;
    for(var i = 0; i < rows; i++){
        arr.push([]);
        arr[i].push( new Array(cols));
        for(var j=0; j < cols; j++){
            arr[i][j] = number;
            number++;
        }
    }
    return arr;
}

function displayInDiv() {
    var output_string_ = "" ;
    var lastElement = 64 ;
    var output_string_ = '<table>'
    for ( var i = 0 ; i < 8 ; i++ ) { 
        output_string_ += "<tr id='table_row_" + i + "'>";
        for ( var j = 0 ; j < 8 ; j++ ) {
            output_string_ += '<td width="11%" align="left">' + x[i][j] + '</td>';
        }
        output_string_ += '</tr>';
    }
    output_string_ += '</table>';

    var output_section_ = document.getElementById("list_");
    output_section_.innerHTML = output_string_ ; 
}

var num;
var arr = [];

// This for loop populates the array
for (var i = 0; i <= 7; i++) {
  arr[i] = [];
  for (var j = 0; j <= 7; j++) {
    arr[i][j] = i * 8 + j + 1;
  }
}

//  This function displays the populated array
function displayArr(arr) {
  var container = document.getElementById("container");
  var append = "<table>";

  for (var i = 0; i < arr.length; i++) {
    append += "<tr>";

    for (var j = 0; j < arr[i].length; j++) {
      num = i * 8 + j + 1;
      append += "<td>" + num + "</td>";
    }

    append += "</tr>";
  }
  
  container.innerHTML = append;
}

//  Alerts the sum of all numbers in the 2d array
function getSum() {
  var sum = 0;
  for (var i = 0; i <= 7; i++) {
    for (var j = 0; j <= 7; j++) {
      sum += i * 8 + j + 1;
    }

  }
  return sum;
}


// Alerts the average of all numbers in the 2d array
function getAvg() {
  var totalNumbers = 0;
  var avg = 0;
  for (var i = 0; i <= 7; i++) {
    for (var j = 0; j <= 7; j++) {
      totalNumbers++;
    }

  }
  return getSum() / totalNumbers;
}


alert(getAvg());
alert(getSum());
displayArr(arr);

<section id="container"></section>

EDIT* Added the average and sum functions