This is a simple demonstration of how then()-catch() methods work while heading an api. We can see clearly then() methods are called after the previous promise resolve.

basic-promise.js

// Here every then() method returns a new promise
// then() method always yields a promise after being executed, this way chaining is possible
// We can say here that every then() methods waits the previous promise to resolve
// Error handling is also easy. If any prior promise fails, catch() method will be triggered
fetch('https://api.openweathermap.org/data/2.5/weather?q=London,uk')
.then(response => {
    return response.json();
})
.then(data => {
    console.log(data);
})
.catch(error => {
    console.log('There is an error!');
    console.log(error);
});