It treats each element in the array as an array, so '456' is treated as an array with the elements 4,5,6.

A strange workaround I did was to loop through the array and put each element inside its own array, so it would be:

var newArray = [];

 for( var i = 0; i < tempArray.length; i++){
 var arr = [];
 arr.push(tempArray[i]);
newArray.push(arr);
}

And then write the "newArray".

But otherwise if you write row by row, you can put each element inside a bracket, like:

var fast_csv = fastcsv.createWriteStream();
var writeStream = fs.createWriteStream("outputfile.csv");
fast_csv.pipe(writeStream);

for(var i = 0; i < tempArray.length; i++){
    fast_csv.write( [ tempArray[i]  ] );             //each element inside bracket
    }
fast_csv.end();