Call stack, event loop and async programming

1. Модель совпадения и цикл событий. Асинхронное выполнение, функции как колбэки, Promise Masters Academy, 2018

5. 1. Стэк (stack) контекстов выполнения функций программы 2. Куча (heap) - неструктурированная область памяти, для хранения объектов 3. Очередь событий (queue) - список событий, подлежащих обработке вследствие выполнения асинхронных операций

6. and more...

7. function foo(x) { return 5 * x; } function bar(y) { return foo(y) - 3; } function baz(z) { return bar(z) + 7; } baz(15); “First in - last out” principle

8. function foo(x) { return 5 * x; } function bar(y) { return foo(y) - 3; } function baz(z) { return bar(z) + 7; } baz(15);

14. setTimeout(function() { console.log(‘timeout 500 complete!’); }, 500); function foo(x) { return 5 * x; } setTimeout(function() { console.log(‘timeout 0 complete!’); }, 0); function baz(z) { console.log(foo(z) + 7) } baz(15);

29. setTimeout(function() { console.log(‘timeout 500 complete!’); }, 500); function foo(x) { return 5 * x; } setTimeout(function() { console.log(‘timeout 0 complete!’); }, 0); function baz(z) { console.log(foo(z) + 7) } baz(15); Output: // 82 // timeout 0 complete // timeout 500 complete

30. Callbacks setTimeout(() => { console.log(‘Hello World!’) }, 1000); ajax(‘http://someurl.com’, ‘GET’, data => console.log(data)) // ajax in some http lib element.addEventListener(‘click’, evt => console.log(‘element clicked!!!’))

31. Callback Hell button.addEventListener(‘click’, (evt) => { ajax(`http://authorize.com/${evt.target.id}`, ‘GET’, res => { ajax(`http://someapi.com/${res}`, ‘GET’, data => { setTimeout(() => { console.log(`finally some data: ${data}`) }, 500) }) }) });

33. Inversion of control / Инверсия контроля

34. Promise const promise = new Promise((resolve, reject) => { ajax(‘http://sample.com’, ‘GET’, done => resolve(done), err => reject(err)) }) promise .then(res => console.log(res)) .catch(err => console.log(‘Error occurred!’)) .finally(() => console.log(‘Hello from resolved/rejected promise!!’))

35. Promise features 1. No inversion of control, you get the future value 2. Immutable after resolve/reject 3. Completes only once (resolve/reject) 4. Chainable, always returns a Promise instance 5. Lighter syntax 6. Utilizes ES6 Jobs (acts like async microtask, not task) 7. Promise.all, Promise.race and more 8. Cannot be cancelled

36. Promise Chaining function createRequest(url) { return new Promise((resolve, reject) => { ajax(url, resolve, reject); // assume ajax is some http library }) } const promise = createRequest(‘http://auth.com’); promise.then(createRequest).then(createRequest).catch(console.log)

37. Handy to use Promise.all // resolves when all promises are resolved Promise.race // resolves when one of promises is resolved fetch // A modern replacement for XMLHttpRequest

38. async functions async function authorizeUser(user, pass) { const authUrl = await createRequest(`http://auth.com/${user}/${pass}`); const userData = await createRequest(authUrl); return userData; } authorizeUser(‘Jon Doe’, ‘very_secure_password’) .then(console.log).catch(console.log);

39. Good resources 1. https://www.youtube.com/watch?v=8aGhZQkoFbQ nice video about call stack and event loop 1. https://medium.com/front-end-hacking/callbacks-promises-and-async-await- ad4756e01d90 callback, Promise, async/await concepts 1. https://jakearchibald.com/2015/tasks-microtasks-queues-and-schedules/ tasks vs microtasks in event loop

40. Homework Create a function logNumbers() which accepts one argument of type ‘number’. After one second log into console this number + 10. After two seconds log into console previous step result * 3. After three seconds log into console previous step result - 20. Provide callback, Promise and async/await based solution.

41. Homework ** (only if you get bored with first one) fetch() is cool for HTTP requests. It’s Promise-based and has lots of options. But as Promise resolves/rejects only once and stays immutable, we cannot use it for reporting file upload progress purposes (PUT). Create a function which would provide a possible solution for this. Hint: this function will not use fetch()

Call stack, event loop and async programming

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