function attachEventsListeners() {

    let [inputField, outputField] = document.querySelector('input[type="text"]');

    let fromOption = document.querySelector('#inputUnits');
    let toOption = document.querySelector('#outputUnits');
    let convertBtn = document.getElementById('convert');

    convertBtn.addEventListener('click', operation);


        function operation() {
            let convert = Number(inputField.value);
            let result = 0;
            switch (fromOption.value) {
                case 'km': convert *= 1000; break;
                case 'm': convert = convert; break;
                case 'cm': convert *= 0.01; break;
                case 'mm': convert *= 0.001; break;
                case 'mi': convert *= 1609.34; break;
                case 'yrd': convert *= 0.9144; break;
                case 'ft': convert *= 0.3048; break;
                case 'in': convert *= 0.0254; break;
            }
            switch (toOption.value) {
                case 'km': result = convert / 1000; break;
                case 'm': result = convert; break;
                case 'cm': result = convert / 0.01; break;
                case 'mm': result = convert / 0.001; break;
                case 'mi': result = convert / 1609.34; break;
                case 'yrd': result = convert / 0.9144; break;
                case 'ft': result = convert / 0.3048; break;
                case 'in': result = convert / 0.0254; break;
            }
            outputField.value = result;
        }



}