SOURCE

alert(navigator.connection.type);

function getNetworkType() {
    let ua = navigator.userAgent;
    let networkStr = ua.match(/NetType\/\w+/) ? ua.match(/NetType\/\w+/)[0] : 'NetType/other';
    networkStr = networkStr.toLowerCase().replace('nettype/', '');
    let networkType;
    switch(networkStr) {
        case 'wifi':
            networkType = 'wifi';
            break;
        case '4g':
            networkType = '4g';
            break;
        case '3g':
            networkType = '3g';
            break;
        case '3gnet':
            networkType = '3g';
            break;
        case '2g':
            networkType = '2g';
            break;
        default:
            networkType = 'other';
    }
    return networkStr;
}

alert(getNetworkType());