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+
+# Callback hell
+
+Many things that we do in JavaScript are asynchronous. We initiate a process, but it finishes later.
+
+The most obvious example is `setTimeout`, but there are others, like making network requests, performing animations and so on.
+
+Let's see a couple of examples, so that we can discover a problem, and then solve it using "promises".
+
+[cut]
+
+## Callbacks
+
+Remember resource load/error events? They are covered in the chapter <info:onload-onerror>.
+
+Let's say we want to create a function `loadScript` that loads a script and executes our code afterwards.
+
+It can look like this:
+
+```js
+function loadScript(src, callback) {
+  let script = document.createElement('script');
+  script.src = src;
+
+  script.onload = () => callback(script);
+
+  document.head.append(script);
+}
+```
+
+Now when we want to load a script and then do something, we can call:
+
+```js
+loadScript('/my/script.js', function(script) {
+  alert(`Cool, the ${script.src} is loaded, let's use it`);
+});
+```
+
+...And it works, shows `alert` after the script is loaded.
+
+That is called "a callback API". Our function `loadScript` performs an asynchronous task and we can hook on its completion using the callback function.
+
+## Callback in callback
+
+What if we need to load two scripts: one more after the first one?
+
+We can put another `loadScript` inside the callback, like this:
+
+```js
+loadScript('/my/script.js', function(script) {
+  alert(`Cool, the ${script.src} is loaded, let's load one more`);
+
+  loadScript('/my/script2.js', function(script) {
+
+    alert(`Cool, the second script is loaded`);
+
+  });
+
+});
+```
+
+Now after the outer `loadScript` is complete, the callback initiates the inner one.
+
+...What if we want one more script?
+
+```js
+loadScript('/my/script.js', function(script) {
+
+  loadScript('/my/script2.js', function(script) {
+
+    if (something) {
+      loadScript('/my/script3.js', function(script) {
+*!*
+        // ...continue after all scripts are loaded
+*/!*
+      });
+    }
+
+  })
+
+});
+```
+
+As you can see, a new asynchronous action means one more nesting level.
+
+## Handling errors
+
+In this example we didn't consider errors. What if a script loading failed with an error? Our callback should be able to react on that.
+
+Here's an improved version of `loadScript` that can handle errors:
+
+```js run
+function loadScript(src, callback) {
+  let script = document.createElement('script');
+  script.src = src;
+
+*!*
+  script.onload = () => callback(null, script);
+  script.onerror = () => callback(new Error(`Script load error ` + src));
+*/!*
+
+  document.head.append(script);
+}
+```
+
+It calls `callback(null, script)` for successful load and `callback(error)` otherwise.
+
+Usage:
+```js
+loadScript('/my/script.js', function(error, script) {
+  if (error) {
+    // handle error
+  } else {
+    // script loaded, go on
+  }
+});
+```
+
+The first argument of `callback` is reserved for errors and the second argument for the successful result. That allows to use a single function to pass both success and failure.
+
+## Pyramid of doom
+
+From the first look it's a viable code. And indeed it is. For one or maybe two nested calls it looks fine.
+
+But for multiple asynchronous actions that follow one after another...
+
+```js
+loadScript('/my/script1.js', function(error, script) {
+
+  if (error) {
+    handleError(error);
+  } else {
+    // ...
+    loadScript('/my/script2.js', function(error, script) {
+      if (error) {
+        handleError(error);
+      } else {
+        // ...
+        loadScript('/my/script3.js', function(error, script) {
+          if (error) {
+            handleError(error);
+          } else {
+  *!*
+            // ...continue after all scripts are loaded (*)
+  */!*
+          }
+        });
+
+      }
+    })
+  }
+});
+```
+
+In the code above:
+1. we load `/my/script1.js`, then if there's no error
+2. we load `/my/script2.js`, then if there's no error
+3. we load `/my/script3.js`, then if there's no error -- do something else `(*)`.
+
+The nested calls become increasingly more difficult to manage, especially if we add real code instead of `...`, that may include more loops, conditional statements and other usage of loaded scripts.
+
+That's sometimes called "callback hell" or "pyramid of doom".
+
+![](callback-hell.png)
+
+See? It grows right with every asynchronous action.
+
+Compare that with a "regular" synchronous code.
+
+Just *if* `loadScript` were a regular synchronous function:
+
+```js
+try {
+  // assume we get the result in a synchronous manner, without callbacks
+  let script = loadScript('/my/script.js');
+  // ...
+  let script2 = loadScript('/my/script2.js');
+  // ...
+  let script3 = loadScript('/my/script3.js');
+} catch(err) {
+  handleError(err);
+}
+```
+
+How much cleaner and simpler it is!
+
+Promises allow to write asynchronous code in a similar way. They are really great at that. Let's study them in the next chapter.
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+# Promise
+
+There are many ways to explain promises. Here we'll follow the [specification](https://tc39.github.io/ecma262/#sec-promise-objects), because gives real understanding how things work. Besides, you'll become familiar with the terms.
+
+First, what a promise is, and then usage patterns.
+
+[cut]
+
+## Promise objects
+
+A promise is an object of the built-in `Promise` class. It has the meaning of the "delayed result".
+
+The constructor syntax is:
+
+```js
+let promise = new Promise(function(resolve, reject) {
+  // ...
+});
+```
+
+The function is called *executor*. It is called automatically when the promise is created and can do an asynchronous job, like loading a script, but can be something else.
+
+At the end, it should call one of:
+
+- `resolve(result)` -- to indicate that the job finished successfully with the `result`.
+- `reject(error)` -- to indicate that an error occured, and `error` should be the `Error` object (technically can be any value).
+
+For instance:
+
+```js
+let promise = new Promise(function(*!*resolve*/!*, reject) {
+  // this function is executed automatically when the promise is constructed
+  setTimeout(() => *!*resolve("done!")*/!*, 1000);
+});
+```
+
+Or, in case of an error:
+
+```js
+let promise = new Promise(function(resolve, *!*reject*/!*) {
+  setTimeout(() => *!*reject(new Error("Woops!"))*/!*, 1000);
+});
+```
+
+Initially, the promise is said to have a "pending" state. Then it has two ways. When `resolve` is called, the state becomes "fulfilled". When `reject` is called, the state becomes "rejected":
+
+![](promiseInit.png)
+
+The idea of promises is that an external code may react on the state change.
+
+The `promise` object  provides the method `promise.then(onResolve, onReject)`.
+
+Both its arguments are functions:
+
+- `onResolve` is called when the state becomes "fulfilled" and gets the result.
+- `onReject` is called when the state becomes "rejected" and gets the error.
+
+For instance, here `promise.then` outputs the result when it comes:
+
+```js run
+let promise = new Promise(function(resolve, reject) {
+  setTimeout(() => resolve("done!"), 1000);
+});
+
+// shows "done!" after 1 second
+promise.then(result => alert(result));
+```
+
+...And here it shows the error message:
+
+
+```js run
+let promise = new Promise(function(resolve, reject) {
+  setTimeout(() => reject(new Error("Woops!")), 1000);
+});
+
+// shows "Woops!" after 1 second
+promise.then(null, error => alert(error.message));
+```
+
+![](promise-resolve-reject.png)
+
+
+- To handle only a result, we can use single argument: `promise.then(onResolve)`
+- To handle only an error, we can use a shorthand method `promise.catch(onReject)` -- it's the same as `promise.then(null, onReject)`.
+
+Here's the example rewritten with `promise.catch`:
+
+```js run
+let promise = new Promise(function(resolve, reject) {
+  setTimeout(() => reject(new Error("Woops!")), 1000);
+});
+
+*!*
+promise.catch(error => alert(error.message));
+*/!*
+```
+
+"What's the benefit?" -- one might ask, "How do we use it?" Let's see an example.
+
+## Example: loadScript
+
+We have the `loadScript` function for loading a script from the previous chapter, here's the callback-based variant:
+
+```js
+function loadScript(src, callback) {
+  let script = document.createElement('script');
+  script.src = src;
+
+  script.onload = () => callback(null, script);
+  script.onerror = () => callback(new Error(`Script load error ` + src));
+
+  document.head.append(script);
+}
+```
+
+Let's rewrite it using promises.
+
+The call to `loadScript(src)` below returns a promise that settles when the loading is complete:
+
+```js run
+function loadScript(src) {  
+  return new Promise(function(resolve, reject) {
+    let script = document.createElement('script');
+    script.src = src;
+
+    script.onload = () => resolve(script);
+    script.onerror = () => reject(new Error("Script load error: " + src));
+
+    document.head.append(script);
+  });
+}
+
+*!*
+// Usage:
+*/!*
+let promise = loadScript("https://cdnjs.cloudflare.com/ajax/libs/lodash.js/3.2.0/lodash.js");
+promise.then(
+  script => alert(`${script.src} is loaded!`),
+  error => alert(`Error: ${error.message}`);
+);
+```
+
+The benefits compared to the callback syntax:
+
+- We can add as many `.then` as we want and when we want. Maybe later.
+- Also we can pass a promise object somewhere else, and new handlers can be added there, so that's extensible.
+
+So promises already give us flexibility. But there's more. We can chain promises, see the next chapter.
+
+
+````warn header="Once a promise settles, it can't be changed"
+When either `resolve` or `reject` is called -- the promise becomes *settled* (fulfilled or rejected), and the state is final. The result is saved in the promise object.
+
+Future calls of `resolve/reject` are ignored, there's no way to "re-resolve" or "re-reject" a promise.
+
+For instance, here only the first `resolve` works:
+
+```js run
+let promise = new Promise(function(resolve, reject) {
+  resolve("done!"); // immediately fulfill with the result: "done"
+
+  setTimeout(() => resolve("..."), 1000); // ignored
+  setTimeout(() => reject(new Error("..."), 2000));  // ignored
+});
+
+// the promise is already fulfilled, so the alert shows up right now
+promise.then(result => alert(result), () => alert("Never runs"));
+````
+
+
+```smart header="On settled promises `then` runs immediately"
+As we've seen from the example above, a promise may call resolve/reject without delay. That happens sometimes, if it turns out that there's no asynchronous things to be done.
+
+When the promise is settled (resolved or rejected), subsequent `promise.then` callbacks are executed immediately.
+```
+
+````smart header="Functions resolve/reject have only one argument"
+Functions `resolve/reject` accept only one argument.
+
+We can call them without any arguments: `resolve()`, that makes the result `undefined`:
+
+```js run
+let promise = new Promise(function(resolve, reject) {
+  resolve();
+});
+
+promise.then(result => alert(result)); // undefined
+```
+
+...But if we pass more arguments: `resolve(1, 2, 3)`, then all arguments after the first one are ignored. A promise may have only one value as a result/error.
+
+Use objects and destructuring if you need to pass many values, like this:
+
+```js run
+let promise = new Promise(function(resolve, reject) {
+  resolve({name: "John", age: 25}); // two values packed in an object
+});
+
+// destructuring the object into name and age variables
+promise.then(function({name, age}) {
+  alert(`${name} ${age}`); // John 25
+})
+```
+
+````
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+<script>
+function loadImage(src) {
+  return new Promise((resolve, reject) => {
+    let img = document.createElement('img');
+    img.src = src;
+    img.onload = () => resolve(img);
+    img.onerror = () => reject(new Error("Image load failed: " + src));
+  });
+}
+</script>
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+development: true
+
+---
+
+# Promises, async/await