Arrow function expressions

An arrow function expression is a compact alternative to a traditional function expression, but is limited and can't be used in all situations.

There are differences between arrow functions and traditional functions, as well as some limitations:

• Arrow functions don't have their own bindings to [this](<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/this>), [arguments](<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/arguments>) or [super](<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/super>), and should not be used as methods.
• Arrow functions don't have access to the [new.target](<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/new.target>) keyword.
• Arrow functions aren't suitable for [call](<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/call>), [apply](<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/apply>) and [bind](<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind>) methods, which generally rely on establishing a scope.
• Arrow functions cannot be used as constructors.
• Arrow functions cannot use [yield](<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/yield>), within its body.

// Traditional Anonymous Function
function (a){
  return a + 100;
}

// Arrow Function Break Down

// 1. Remove the word "function" and place arrow between the argument and opening body bracket
(a) => {
  return a + 100;
}

// 2. Remove the body braces and word "return" -- the return is implied.
(a) => a + 100;

// 3. Remove the argument parentheses
a => a + 100;

Promises

A Promise is a proxy for a value not necessarily known when the promise is created. It allows you to associate handlers with an asynchronous action's eventual success value or failure reason. This lets asynchronous methods return values like synchronous methods: instead of immediately returning the final value, the asynchronous method returns a promise to supply the value at some point in the future.

A Promise is in one of these states:

• pending: initial state, neither fulfilled nor rejected.
• fulfilled: meaning that the operation was completed successfully.
• rejected: meaning that the operation failed.

A pending promise can either be fulfilled with a value or rejected with a reason (error). When either of these options happens, the associated handlers queued up by a promise's then method are called. If the promise has already been fulfilled or rejected when a corresponding handler is attached, the handler will be called, so there is no race condition between an asynchronous operation completing and its handlers being attached.

As the [Promise.prototype.then()](<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/then>) and [Promise.prototype.catch()](<https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Promise/catch>) methods return promises, they can be chained.

const myPromise = new Promise((resolve, reject) => {
  setTimeout(() => {
    resolve('foo');
  }, 300);
});

myPromise
  .then(handleResolvedA, handleRejectedA)
  .then(handleResolvedB, handleRejectedB)
  .then(handleResolvedC, handleRejectedC);