Convert 2D JS array to json string

My array in variable is:

"1": ["48": '1', "49": '2']
"2": ["51": '3', "52": '4', "53": '5', "54": '6']
"3": ["30": '7']

I’ve mentioned key and value here for the 2D array, I’m trying to convert this to JSON string. I tried JSON.stringify(arraydata), arraydata is the variable where the array is stored, but it makes the string empty, whereas array data is correct.

Edit: This is how I’m adding array data:

var arraydata = new Array();
$('.classselector').each(function(){
    let key1= $(this).data('key1');
    let key2= $(this).data('key2');
    if ( !Array.isArray(arraydata['"'+key1+'"']) ) {
        arraydata['"'+key1+'"'] = new Array();
    }
    arraydata['"'+key1+'"']['"'+key2+'"'] = $(this).val();      
});

Answer

The “array” quoted in your question is not valid JavaScript code. Maybe you had on object of objects instead? In that case the object can easily be converted into a JSON string:

const obj={"1": {"48": '1', "49": '2'},
"2": {"51": '3', "52": '4', "53": '5', "54": '6'},
"3": {"30": '7'}};

console.log(JSON.stringify(obj));

// in case you were really talking about 
// a sparsely populated array of arrays,
// then the solution could look like this:

const arr=[];
arr[1]=[];
arr[1][48]='1';
arr[1][49]='2';
arr[2]=[];
arr[2][51]='3';
arr[2][52]='4';
arr[2][53]='5';
arr[2][54]='6';
arr[3]=[];
arr[3][30]='7';

console.log(arr);
console.log(JSON.stringify(arr));

See my comment above. Use objects instead of arrays!

Your corrected script could look something like this:

var arraydata = {};
$('.classselector').each(function(){
    let key1= $(this).data('key1');
    let key2= $(this).data('key2');
    if ( !arraydata[key1] ) {
        arraydata[key1] = {};
    }
    arraydata[key1][key2] = $(this).val();      
});