JsStore provides you a way to update data with arithmetic operators.

e.g - you want to add 5 to current stored data.

connection.update({
    in: "Table_Name",
    set: {
        Column1: {
            '+': value1
        },
        //Supported operators are - +, -, *, /
        Column2: value2
    },
    where: {
        Column3: some_value,
        Column4: some_another_value
    }
}).then(function(rowsUpdated) {
    alert(rowsUpdated + ' rows updated');
}).catch(function(err) {
    console.log(err);
});

Example