Lesson Arrow functions, promises and async / await

lesson5.js

// Arrow functions

let addFunction = function(a, b) {
  return a + b;
}

// these are different ways to write functions, with only very minor differences
addFunction =  function(a, b)    { return a + b };
addFunction =          (a, b) => { return a + b };
addFunction =          (a, b) =>          a + b  ;
addFunction =           a     =>          a + 1  ;


console.log(addFunction(4, 18));


// Most common arrow function
addFunction = (a, b) => {
  // some extra stuff happens here
  return a + b;
};

// which is the same as
addFunction = function(a, b) {
  // some extra stuff happens here
  return a + b;
}



// Promises
let promise = new Promise(function(resolve, reject) {
  setTimeout(function() {
    resolve("done!");
  }, 2000);
});

console.log(promise);

promise.then(function(data) {
  console.log(data)
  console.log(promise);
});


// Arrow functions make things easier!

let promise = new Promise((resolve, reject) => {
  setTimeout(() => resolve("done!"), 2000);
});

console.log(promise);

promise.then(data => {
  console.log(data)
  console.log(promise);
});


// We can also reject!
let promise = new Promise((resolve, reject) => {
  setTimeout(() => reject("rejected!"), 2000);
});

console.log(promise);

promise.catch(data => {
  console.log(data)
  console.log(promise);
});



// If you want to do something after a promise is no longer pending but you don't know if it will be resolved or rejected you can use 'finally'
// Feel free to use .then, .catch and .finally in a combination, you can chain them like this:
promise
  .then(data => console.log(`Success: ${data}`))
  .catch(data => console.log(`Failure: ${data}`))
  .finally(() => console.log('We\'re done with this promise :)'));



// Chained promises
// Sometimes you wait for a promise to resolve, which will then fire another promise, and then you wait for this, and you get something like this:

let myPromise = new Promise((resolve, reject) => {
  setTimeout(() => resolve('done'), 2000);
});

let myOtherPromise = new Promise((resolve, reject) => {
  setTimeout(() => reject(new Error('Something went wrong oh no :\'(')), 4000);
});

console.log('Start!');
myPromise.then(() => {
  myOtherPromise.then(result => {
    console.log(result);
  }).catch(error => {
    // Alternatively, remove the catch block to let the error throw and terminate your program
    console.warn(error);
  }).finally(() => console.log('finally done with these promises'));
});

// Insert AND THEN gif

// We have a nice method of making this easier called async / await

mySynchronousFunction = () => {
  // do something
}

myAsynchronousFunction = async () => {
  console.log('Start!');
  await myPromise;
  try {
    let result = await myOtherPromise;
  } catch (error) {
    // Alternatively, remove the catch block to let the error throw and terminate your program
    console.warn(error);
  } finally {
    console.log('finally done with all these promises!');
  }
}

myAsynchronousFunction();