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In this lesson we'll teach you how to write asynchronous JavaScript using native promises.

JavaScript Promises

Knowing how to construct a promise is useful, but most of the time, knowing how to _consume_, or use, promises will be key. Rather than constructing promises, you'll be handling `Promise` objects returned to you as the result of an asynchronous operation. These promises will start off pending but settle eventually.

Moving forward, we'll be simulating this by providing you with functions that return promises which settle after some time. To accomplish this, we'll be using `setTimeout()`. `setTimeout()` is a Node API (a comparable API is provided by web browsers) that uses callback functions to schedule tasks to be performed after a delay. `setTimeout()` has two parameters: a callback function and a delay in milliseconds. 

```js
const delayedHello = () => {
  console.log('Hi! This is an asynchronous greeting!');
};

setTimeout(delayedHello, 2000);
```
Here, we invoke `setTimeout()` with the callback function `delayedHello()` and `2000`. In at least two seconds `delayedHello()` will be invoked. But why is it "at least" two seconds and not exactly two seconds? 

This delay is performed asynchronously—the rest of our program won't stop executing during the delay. Asynchronous JavaScript uses something called _the event-loop_. After two seconds, `delayedHello()` is added to a line of code waiting to be run. Before it can run, any synchronous code from the program will run. Next, any code in front of it in the line will run. This means it might be more than two seconds before `delayedHello()` is actually executed. 

Let's look at how we'll be using `setTimeout()` to construct asynchronous promises:
```js
const returnPromiseFunction = () => {
  return new Promise((resolve, reject) => {
    setTimeout(( ) => {resolve('I resolved!')}, 1000);
  });
};

const prom = returnPromiseFunction();
```
In the example code, we invoked `returnPromiseFunction()` which returned a promise. We assigned that promise to the variable `prom`. Similar to the asynchronous promises you may encounter in production, `prom` will initially have a status of pending.

Let's explore `setTimeout()` a bit more. 

The Node setTimeout() Function

diagram of a running dishwasher and its possible outcomes 

Promises are objects that represent the eventual outcome of an asynchronous operation. A `Promise` object can be in one of three states:
* **Pending**: The initial state&mdash; the operation has not completed yet.
* **Fulfilled**: The operation has completed successfully and the promise now has a _resolved value_. For example, a request's promise might resolve with a JSON object as its value.
* **Rejected**: The operation has failed and the promise has a reason for the failure. This reason is usually an `Error` of some kind.

We refer to a promise as _settled_ if it is no longer pending&mdash; it is either fulfilled or rejected. Let's think of a dishwasher as having the states of a promise:

* **Pending**: The dishwasher is running but has not completed the washing cycle.
* **Fulfilled**: The dishwasher has completed the washing cycle and is full of clean dishes.
* **Rejected**: The dishwasher encountered a problem (it didn't receive soap!) and returns unclean dishes.  

If our dishwashing promise is fulfilled, we'll be able to perform related tasks, such as unloading the clean dishes from the dishwasher. If it's rejected, we can take alternate steps, such as running it again with soap or washing the dishes by hand. 

All promises eventually settle, enabling us to write logic for what to do if the promise fulfills or if it rejects. 

What is a Promise?

When done correctly, promise composition is a great way to handle situations where asynchronous operations depend on each other or execution order matters. What if we're dealing with multiple promises, but we don't care about the order? Let's think in terms of cleaning again. 

For us to consider our house clean, we need our clothes to dry, our trash bins emptied, and the dishwasher to run. We need **all** of these tasks to complete but not in any particular order. Furthermore, since they're all getting done asynchronously, they should really all be happening at the same time! 

To maximize efficiency we should use _concurrency_, multiple asynchronous operations happening together. With promises, we can do this with the function `Promise.all()`. 

`Promise.all()` accepts an array of promises as its argument and returns a single promise. That single promise will settle in one of two ways: 
+ If every promise in the argument array resolves, the single promise returned from `Promise.all()` will resolve with an array containing the resolve value from each promise in the argument array.
+ If any promise from the argument array rejects, the single promise returned from `Promise.all()` will immediately reject with the reason that promise rejected. This behavior is sometimes referred to as _failing fast_. 

Let's look at a code example:
```js
let myPromises = Promise.all([returnsPromOne(), returnsPromTwo(), returnsPromThree()]);

myPromises
  .then((arrayOfValues) => {
    console.log(arrayOfValues);
  })
  .catch((rejectionReason) => {
    console.log(rejectionReason);
  });
```
Let's break down what's happening:
+ We declare `myPromises` assigned to invoking `Promise.all()`.
+ We invoke `Promise.all()` with an array of three promises&mdash; the returned values from functions.
+ We invoke `.then()` with a success handler which will print the array of resolved values if each promise resolves successfully. 
+ We invoke `.catch()` with a failure handler which will print the first rejection message if any promise rejects. 

 

Using Promise.all()

One common pattern we'll see with asynchronous programming is multiple operations which depend on each other to execute or that must be executed in a certain order. We might make one request to a database and use the data returned to us to make another request and so on! Let's illustrate this with another cleaning example, washing clothes: 

We take our dirty clothes and put them in the washing machine. If the clothes are cleaned, **then** we'll want to put them in the dryer. After the dryer runs, if the clothes are dry, **then** we can fold them and put them away. 

This process of chaining promises together is called _composition_. Promises are designed with composition in mind! Here's a simple promise chain in code:

```js
firstPromiseFunction()
.then((firstResolveVal) => {
  return secondPromiseFunction(firstResolveVal);
})
.then((secondResolveVal) => {
  console.log(secondResolveVal);
});
```
Let's break down what's happening in the example:
+ We invoke a function `firstPromiseFunction()` which returns a promise. 
+ We invoke `.then()` with an anonymous function as the success handler.
+ Inside the success handler we **return** a new promise&mdash; the result of invoking a second function, `secondPromiseFunction()` with the first promise's resolved value.
+ We invoke a second `.then()` to handle the logic for the second promise settling. 
+ Inside that `.then()`, we have a success handler which will log the second promise's resolved value to the console. 

In order for our chain to work properly, we had to `return` the promise `secondPromiseFunction(firstResolveVal)`. This ensured that the return value of the first `.then()` was our second promise rather than the default return of a new promise with the same settled value as the initial. 

Let's write some promise chains!

Chaining Multiple Promises

In web development, asynchronous programming is notorious for being a challenging topic. 

An _asynchronous operation_ is one that allows the computer to "move on" to other tasks while waiting for the asynchronous operation to complete. Asynchronous programming means that time-consuming operations don't have to bring everything else in our programs to a halt. 
 
There are countless examples of asynchronicity in our everyday lives. Cleaning our house, for example, involves asynchronous operations such as a dishwasher washing our dishes or a washing machine washing our clothes. While we wait on the completion of those operations, we're free to do other chores.

Similarly, web development makes use of asynchronous operations. Operations like making a network request or querying a database can be time-consuming, but JavaScript allows us to execute other tasks while awaiting their completion. 

This lesson will teach you how modern JavaScript handles asynchronicity using the `Promise` object, introduced with ES6. Let's get started!

a person cleaning their kitchen with a cute dog underfoot

Introduction

Awesome job! Promises are a difficult concept even for experienced developers, so pat yourself on the back. You've learned a ton about asynchronous JavaScript and promises. Let's review:
+ Promises are JavaScript objects that represent the eventual result of an asynchronous operation.
+ Promises can be in one of three states: pending, resolved, or rejected.
+ A promise is settled if it is either resolved or rejected.
+ We construct a promise by using the `new` keyword and passing an executor function to the `Promise` constructor method.
+ `setTimeout()` is a Node function which delays the execution of a callback function using the event-loop. 
+ We use `.then()` with a success handler callback containing the logic for what should happen if a promise resolves. 
+ We use `.catch()` with a failure handler callback containing the logic for what should happen if a promise rejects. 
+ Promise composition enables us to write complex, asynchronous code that's still readable. We do this by chaining multiple `.then()`'s and `.catch()`'s.
+ To use promise composition correctly, we have to remember to `return` promises constructed within a `.then()`.
+ We should chain multiple promises rather than nesting them. 
+ To take advantage of concurrency, we can use `Promise.all()`.


Review

Let’s construct a promise! To create a new `Promise` object, we use the `new` keyword and the `Promise` constructor method:
```js
const executorFunction = (resolve, reject) => { };
const myFirstPromise = new Promise(executorFunction);
```
The `Promise` constructor method takes a function parameter called the _executor function_ which runs automatically when the constructor is called. The executor function generally starts an asynchronous operation and dictates how the promise should be settled. 

The executor function has two function parameters, usually referred to as the `resolve()` and `reject()` functions. The `resolve()` and `reject()` functions aren't defined by the programmer. When the `Promise` constructor runs, JavaScript will pass **its own** `resolve()` and `reject()` functions into the executor function.

* `resolve` is a function with one argument. Under the hood, if invoked, `resolve()` will change the promise's status from `pending` to `fulfilled`, and the promise's resolved value will be set to the argument passed into `resolve()`.
* `reject` is a function that takes a reason or error as an argument. Under the hood, if invoked, `reject()` will change the promise's status from `pending` to `rejected`, and the promise's rejection reason will be set to the argument passed into `reject()`.

Let's look at an example executor function in a `Promise` constructor:
 ```js
const executorFunction = (resolve, reject) => {
  if (someCondition) {
      resolve('I resolved!');
  } else {
      reject('I rejected!'); 
  }
}
const myFirstPromise = new Promise(executorFunction);
```
Let's break down what's happening above:
+ We declare a variable `myFirstPromise` 
+ `myFirstPromise` is constructed using `new Promise()` which is the `Promise` constructor method.
+ `executorFunction()` is passed to the constructor and has two functions as parameters: `resolve` and `reject`. 
+ If `someCondition` evaluates to `true`, we invoke `resolve()` with the string `'I resolved!'`
+ If not, we invoke `reject()` with the string `'I rejected!'`

In our example, `myFirstPromise` resolves or rejects based on a simple condition, but, in practice, promises settle based on the results of asynchronous operations. For example, a database request may fulfill with the data from a query or reject with an error thrown. In this exercise, we'll construct promises which resolve synchronously to more easily understand how they work.

Constructing a Promise Object

Promise composition allows for much more readable code than the nested callback syntax that preceded it. However, it can still be easy to make mistakes. In this exercise, we'll go over two common mistakes with promise composition. 

**Mistake 1:** Nesting promises instead of chaining them. 
```js
returnsFirstPromise()
.then((firstResolveVal) => {
  return returnsSecondValue(firstResolveVal)
    .then((secondResolveVal) => {
      console.log(secondResolveVal);
    })
})
```
Let's break down what's happening in the above code:
+ We invoke `returnsFirstPromise()` which returns a promise. 
+ We invoke `.then()` with a success handler.
+ Inside the success handler, we invoke `returnsSecondValue()` with `firstResolveVal` which will return a new promise.
+ We invoke a second `.then()` to handle the logic for the second promise settling all **inside** the first `then()`! 
+ Inside that second `.then()`, we have a success handler which will log the second promise's resolved value to the console. 

Instead of having a clean chain of promises, we've nested the logic for one inside the logic of the other. Imagine if we were handling five or ten promises!

**Mistake 2:** Forgetting to `return` a promise.
```js
returnsFirstPromise()
.then((firstResolveVal) => {
  returnsSecondValue(firstResolveVal)
})
.then((someVal) => {
  console.log(someVal);
})
```
Let's break down what's happening in the example:
+ We invoke `returnsFirstPromise()` which returns a promise. 
+ We invoke `.then()` with a success handler.
+ Inside the success handler, we create our second promise, but we forget to `return` it!
+ We invoke a second `.then()`. It's supposed to handle the logic for the second promise, but since we didn't return, this `.then()` is invoked on a promise with the same settled value as the original promise!

Since forgetting to return our promise won't throw an error, this can be a really tricky thing to debug!

Avoiding Common Mistakes

One way to write cleaner code is to follow a principle called _separation of concerns_. Separation of concerns means organizing code into distinct sections each handling a specific task. It enables us to quickly navigate our code and know where to look if something isn't working. 

Remember, `.then()` will return a promise with the same settled value as the promise it was called on if no appropriate handler was provided. This implementation allows us to separate our resolved logic from our rejected logic. Instead of passing both handlers into one `.then()`, we can chain a second `.then()` with a failure handler to a first `.then()` with a success handler and both cases will be handled. 
 ```js
prom
  .then((resolvedValue) => {
    console.log(resolvedValue);
  })
  .then(null, (rejectionReason) => {
    console.log(rejectionReason);
  });
```
Since JavaScript doesn't mind whitespace, we follow a common convention of putting each part of this chain on a new line to make it easier to read. To create even more readable code, we can use a different promise function: `.catch()`. 

The `.catch()` function takes only one argument, `onRejected`. In the case of a rejected promise, this failure handler will be invoked with the reason for rejection. Using `.catch()` accomplishes the same thing as using a `.then()` with only a failure handler. 

Let's look at an example using `.catch()`:
 ```js
prom
  .then((resolvedValue) => {
    console.log(resolvedValue);
  })
  .catch((rejectionReason) => {
    console.log(rejectionReason);
  });
```
Let's break down what's happening in the example code:
+ `prom` is a promise which randomly either resolves with `'Yay!'` or rejects with `'Ohhh noooo!'`.
+ We pass a success handler to `.then()` and a failure handler to `.catch()`.
+ If the promise resolves, `.then()`'s success handler will be invoked with `'Yay!'`. 
+ If the promise rejects, `.then()` will return a promise with the same rejection reason as the original promise and `.catch()`'s failure handler will be invoked with that rejection reason.

Let's practice writing `.catch()` functions.

Using catch() with Promises

The initial state of an asynchronous promise is `pending`, but we have a guarantee that it will settle. How do we tell the computer what should happen then? Promise objects come with an aptly named `.then()` method. It allows us to say, "I have a promise, when it settles, **then**  here's what I want to happen..."

In the case of our dishwasher promise, the dishwasher will run **then**:
+ If our promise rejects, this means we have dirty dishes, and we'll add soap and run the dishwasher again.
+ If our promise fulfills, this means we have clean dishes, and we'll put the dishes away.

`.then()` is a higher-order function&mdash; it takes two callback functions as arguments. We refer to these callbacks as _handlers_. When the promise settles, the appropriate handler will be invoked with that settled value.
+ The first handler, sometimes called `onFulfilled`, is a _success handler_, and it should contain the logic for the promise resolving. 
+ The second handler, sometimes called `onRejected`, is a _failure handler_, and it should contain the logic for the promise rejecting. 

We can invoke `.then()` with one, both, or neither handler! This allows for flexibility, but it can also make for tricky debugging. If the appropriate handler is not provided, instead of throwing an error, `.then()` will just return a promise with the same settled value as the promise it was called on. One important feature of `.then()` is that it always returns a promise. We'll return to this in more detail in a later exercise and explore why it's so important. 

Consuming Promises

To handle a "successful" promise, or a promise that resolved, we invoke `.then()` on the promise, passing in a success handler callback function:

```js
const prom = new Promise((resolve, reject) => {
  resolve('Yay!');
});

const handleSuccess = (resolvedValue) => {
  console.log(resolvedValue);
};

prom.then(handleSuccess); // Prints: 'Yay!'
```

Let's break down what's happening in the example code:
+ `prom` is a promise which will resolve to `'Yay!'`.
+ We define a function, `handleSuccess()`, which prints the argument passed to it.
+ We invoke `prom`'s `.then()` function passing in our `handleSuccess()` function.
+ Since `prom` resolves, `handleSuccess()` is invoked with `prom`'s resolved value, `'Yay'`, so `'Yay'` is logged to the console.

With typical promise consumption, we won't know whether a promise will resolve or reject, so we'll need to provide the logic for either case. We can pass both a success callback and a failure callback to `.then()`. 

```js
let prom = new Promise((resolve, reject) => {
  let num = Math.random();
  if (num < .5 ){
    resolve('Yay!');
  } else {
    reject('Ohhh noooo!');
  }
});

const handleSuccess = (resolvedValue) => {
  console.log(resolvedValue);
};

const handleFailure = (rejectionReason) => {
  console.log(rejectionReason);
};

prom.then(handleSuccess, handleFailure);
```

Let's break down what's happening in the example code:
+ `prom` is a promise which will randomly either resolve with `'Yay!'` or reject with `'Ohhh noooo!'`.
+ We pass two handler functions to `.then()`. The first will be invoked with `'Yay!'` if the promise resolves, and the second will be invoked with `'Ohhh noooo!'` if the promise rejects.

> Note: The success callback is sometimes called the "success handler function" or the `onFulfilled` function. The failure callback is sometimes called the “failure handler function” or the `onRejected` function. 

Let's write some success and failure callbacks! 