Cannot read property '0' of undefined in jquery

Question

<!DOCTYPE html>
<html>
<head>
<script src="https://ajax.googleapis.com/ajax/libs/jquery/3.1.0/jquery.min.js"></script>
<meta charset="ISO-8859-1">
<title>Insert title here</title>
<style>
td
{
    min-height: 50px;
    min-width: 50px;
}
</style>
</head>
<body>
<table border="1" id="tab">
<tr>
    <td class="game"></td>
    <td class="game"></td>
    <td class="game"></td>
    <td class="game"></td>
</tr>
<tr>
    <td class="game"></td>
    <td class="game"></td>
    <td class="game"></td>
    <td class="game"></td>
</tr>
<tr>
    <td class="game"></td>
    <td class="game"></td>
    <td class="game"></td>
    <td class="game"></td>
</tr>
<tr>
    <td class="game"></td>
    <td class="game"></td>
    <td class="game"></td>
    <td class="game"></td>
</tr>
</table>
<script>
    var tab=[4];
    $(document).ready(function()
    {
        var i,j;
        for(i=0;i<4;i++)
        {
            tab[i]=[4];
            for(j=0;j<4;j++)
            {
                tab[i][j]=null;
            }
        }
        randomnum();
    });
    function randomnum()
    {
        var num=Math.random();
        alert("called random num");
        if(num<0.5)
            num=2;
        else
            num=4;
        alert(num);
        var row=Math.floor(Math.random()*10);
        row=row%4;
        var col=Math.floor(Math.random()*10);
        col=col%4;
        while(tab[row][col]!=null)
        {
            var row=Math.random();
            row=(row*10)%4;
            var col=math.random();
            col=(col*10)%4;
            alert("random row col"+row+" "+col);
        }
        //alert("row:"+row+"col"+col);
        tab[row][col]=num;
        $("#tab tr:eq("+row+") td:eq("+col+")").text(num);
        keycheck();
    }
    function keycheck()
    {
        $(document).on("keydown",function(event){
            if(event.which==38)
                moveup();
            else if(event.which==40)
                movedown();
            else if(event.which==39)
                moveright();
            else if(event.which==37)
                moveleft();
            });
        }
    function moveup()
    {
        var row,col,j;
        for(col=0;col<4;col++)
        {
            for(row=0;row<3;row++)
            {
                if(tab[row][col]==tab[row+1][col])
                {
                    tab[row][col]=tab[row][col]*2;
                    row++;
                    tab[row][col]=null;
                }
            }
            for(row=0;row<3;row++)
            {
                for(j=row+1;j<4;j++)
                {
                    if (typeof j === "undefined") {
                        alert("j is undefined");
                    }
                    if (typeof row === "undefined") {
                        alert("col is undefined");
                    }
                    if (typeof col === "undefined") {
                        alert("col is undefined");
                    }
                    alert(j+" "+row+" "+col);
                    if(tab[j][col]==null&&tab[j+1][col]!=null)
                    {
                        tab[j][col]=tab[j+1][col];
                        tab[j+1][col]=null;
                        j++;
                    }
                }
            }
        }
        chntable();
    }
    function chntable()
    {
        var row,col;
        for(row=0;row<4;row++)
        {
            for(col=0;col<4;col++)
            {
                $("#tab tr:eq("+row+") td:eq("+col+")").text(num);
            }
        }
        randomnum();
    }
</script>
</body>
</html>

in the above code i get the

Cannot read property '0' of undefined

at moveup() in if(tab[j+1][col]==null&&tab[j][col]!=null). what is the error and how to solve it? from the alert messages, I could see that none are undefined. So what is triggering the problem? What is the cause of the problem, so I can avoid it in the future? What is given in duplicate is to create an array, although it did not solve my problem. The error in the jquery library is at dispatch and q.handle.

`tab` has only one key which is `index(0)` and it is holding the value `4`... — Rayon, Jul 30 '16 at 08:46
@Rayon why wasn't the error thrown at any other tab calls and how to solve it — Narayanasami N, Jul 30 '16 at 08:48
@Rayon is there any other way to create a static 4X4 array in jquery — Narayanasami N, Jul 30 '16 at 08:50
Why do you relate array with `jQuery` ? Creating a matrix is quiet easy.. I am unable to understand what you are trying to achieve with your code... — Rayon, Jul 30 '16 at 08:52
@Rayon i am trying to create the [2048 game]http://2048game.com/. so i want a 4X4 array to store the values. — Narayanasami N, Jul 30 '16 at 08:53
Possible duplicate of [JavaScript multidimensional array](http://stackoverflow.com/questions/7545641/javascript-multidimensional-array) — Asons, Jul 30 '16 at 09:09

laren0815 · Answer 1 · 2016-07-30T09:04:23.347

0

tab is an array. In this array you have objects.

Cannot read property '0' of undefined means that you are trying to read a property of an undefined object->

var foo={bar:"test"}
console.log(foo[bar]); //you get "test"

in your case:

var obj = tab[0] //obj is undefined
console.log(obj[0]); //you get "Cannot read property '0' of undefined"

if you want to check, you can easy check every object by using:

if(tab[0]){
   //tab[0] is defined
     if(tab[0][0])
     {
       //tab[0][0] is defined
     }
}

edited Jul 30 '16 at 09:04

answered Jul 30 '16 at 08:54

laren0815

265
1
5
22

is there a way to create an 4X4 array that does not have this problem or is there a workaround for this problem. – Narayanasami N Jul 30 '16 at 08:57
you have to check if the obj is undefined: if(tab[0]) {//tab[0] is defined} – laren0815 Jul 30 '16 at 09:00
i know how to check. i want to know how to solve this so that i dont get any errors – Narayanasami N Jul 30 '16 at 09:07
what´s about if(!tab[0]){tab[0]=randomNumber}... if(!tab[0][0]){tab[0][0]=otherRandomNumber} – laren0815 Jul 30 '16 at 09:17

Cannot read property '0' of undefined in jquery

1 Answers1