HTMLify

let button = document.getElementById('btn');

button.addEventListener('click', function(){
    const gram = parseInt(document.getElementById('grams').value);
    const type = document.getElementById('type').value;

    if(gram === '' || isNaN(gram)){
        document.getElementById('grams').focus();
        document.getElementById('error').innerHTML = 'Please provide a valid gram';
        document.getElementById('output').innerHTML = '';
    }else{
        document.getElementById('error').innerHTML = '';
        switch(type){
            case 'Pounds':
                convert_pounds(gram);
                break;
            case 'Kilograms':
                convert_kilogram(gram);
                break;
            case 'Ounces':
                convert_ounces(gram);
                break;
            case 'Stones':
                convert_stones(gram);
                break;
            default:
                alert('Error');
        }

        function convert_pounds(gram){
            let rate = 0.0022, pounds;
            pounds = gram * rate;
            document.getElementById('output').innerHTML = gram + " grams = " + pounds.toFixed(3) + ' pounds.';
        }
        function convert_kilogram(gram){
            let rate = 0.001, kilogram;
            kilogram = gram * rate;
            document.getElementById('output').innerHTML = gram + " grams = " + kilogram.toFixed(3) + ' kg.';
        }
        function convert_ounces(gram){
            let rate = 0.035274, ounces;
            ounces = gram * rate;
            document.getElementById('output').innerHTML = gram + " grams = " + ounces.toFixed(3) + ' ounces.';
        }
        function convert_stones(gram){
            let rate = 0.00015747, stones;
            stones = gram * rate;
            document.getElementById('output').innerHTML = gram + " grams = " + stones.toFixed(3) + ' stones.';
        }
    }
});