Can anyone tell me why this code

handleCDC(response) {
console.log(JSON.stringify(response));

}

subscribe(channel, -1, handleCDC).then(response=>{
    console.log('success', JSON.stringify(response.channel));
});    

is illegal? why I can't call subscribe method directly?

Can anyone tell me why this code

handleCDC(response) {
    console.log(JSON.stringify(response));
}

subscribe(channel, -1, handleCDC).then(response=>{
    console.log('success', JSON.stringify(response.channel));
});    

is illegal? why I can't call subscribe method directly?