Example 1

Key Concept: A Promise can only change its state once

const myPromise = new Promise((fulfill, reject) => {
    reject();
    fulfill();
});

myPromise.then(
    () => console.log('Success'),
    () => console.log('Failure')
);

Output:

Failure

Example 2

Key Concept: resolve() and reject() do not terminate code eexcution

const promiseInstance = new Promise((resolve, reject) => {
    console.log(1);
    resolve();
    console.log(2);
});

promiseInstance.then(() => {
    console.log(3);
});

Output:

1
2
3

Example 3

Key Concepts:

1. Promise.resolve() is equivalent to new Promise((resolve, reject) => resolve(value))
2. When no error occurs, catch() callback are not invoked

Promise.resolve(1)
    .then(result => 2)
    .catch(error => 3)
    .then(result => console.log(result));

Output:

2

Example 4

Key Concept: Returning a normal valuee from catch() transitions to the next success handler

Promise.resolve(1)
    .then(value => value + 1)
    .then(value => { throw new Error('Custom Error') })
    .catch(() => { throw 'error message' })
    .then(value => value + 1)
    .then(value => console.log(value))
    .catch(error => console.error(error));

Output:

error message

Example 5

Key Concept: Code after await exceutes similarly too promise.then(() => { ... })

async function firstAsync() {
    console.log('firstAsync start');
    await secondAsync();
    console.log('after await');
}

async function secondAsync() {
    console.log('secondAsync');
}

console.log('script start');
setTimeout(() => {
    console.log('timeout');
}, 0);

firstAsync();

new Promise((resolve) => {
    console.log('promise init');
    resolve();
}).then(() => {
    console.log('promise then');
});

console.log('script end');

Output:

script start
firstAsync start
secondAsync
promise init
script end
after await
promise then
timeout