Continuation synchronous and asynchronous

Callbacks can be used to provide code to be executed after a method has completed:

/**
 * @arg {Function} then continuation callback
 */
function doSomething(then) {
  console.log('Doing something');
  then();
}

// Do something, then execute callback to log 'done'
doSomething(function () {
  console.log('Done');
});

console.log('Doing something else');

// Outputs:
//   "Doing something"
//   "Done"
//   "Doing something else"

The doSomething() method above executes synchronously with the callback - execution blocks until doSomething() returns, ensuring that the callback is executed before the interpreter moves on.

Callbacks can also be used to execute code asynchronously:

doSomethingAsync(then) {
  setTimeout(then, 1000);
  console.log('Doing something asynchronously');
}

doSomethingAsync(function() {
  console.log('Done');
});

console.log('Doing something else');

// Outputs:
//   "Doing something asynchronously"
//   "Doing something else"
//   "Done"

The then callbacks are considered continuations of the doSomething() methods. Providing a callback as the last instruction in a function is called a tail-call, which is optimized by ES2015 interpreters.

Found a mistake? Have a question or improvement idea? Let me know.

Callbacks:

* Simple Callback Usage Examples

* What is a callback

* Continuation synchronous and asynchronous

* Callbacks and this

* Error handling and control-flow branching

* Callback using arrow function

Table Of Contents

0 Getting started

1 Comments

2 Basic data types

3 Built-in constants

4 Variables

5 Declarations and assignments

6 Reserved keywords

7 Data types in JavaScript

8 Strings

9 Escape sequences

10 Template Literals

11 Arrays

12 Objects

13 Prototypes objects

14 Classes

15 Variable coercion / conversion

16 Map

17 Set

18 Arithmetic Math

19 Bitwise operators

20 Bitwise operators

21 Unary Operators

22 Comparison operations

23 Conditions

24 Loops

25 Functions

26 Arrow functions

27 Date

28 Date comparison

29 Scope

30 AJAX

31 Promises

32 Regular Expression

33 Error Handling

34 Geolocation

35 Cookies

36 Intervals and timeouts

37 Generators

38 History of visited urls

39 Strict mode

40 Custom Elements

41 JSON

42 Binary Data

43 Web Storage

44 Fetch

45 Modules

46 Screen

47 Inheritance

48 Timestamps

49 Destructuring assignment

50 Web Worker

51 Debugging

52 Notifications API

53 WebSockets

54 Web Cryptography API

55 Async functions, async / await

56 Constructor functions

57 execCommand and contenteditable

58 Performance Tips

59 Design patterns

60 requestAnimationFrame

61 Method chaining

62 Global error handling in browsers

63 File API, blobs and FileReaders

64 Console

65 Tail call optimization

66 Detecting browser

67 Enumerations

68 Symbols

69 Localization

70 Selection API

71 Callbacks

72 Functional JavaScript

73 Modals

74 Data attributes

75 Event Loop

76 Events

77 Data manipulation

78 BOM Browser Object Model

79 Unit Testing

80 Linters for ensuring code quality

81 Automatic Semicolon Insertion

82 IndexedDB

83 Anti-patterns

84 Navigator Object

85 Modularization techniques

86 Proxy

87 Same origin policy, cross-origin communication

88 postMessage and MessageEvent

89 WeakMap

90 WeakSet

91 Behaviroal Design Patterns

92 Server-sent events

93 Async iterators

94 Namespacing

95 Evaluating JavaScript

96 Memory efficiency

97 How to make iterator usable inside async callback function

98 Context this

99 Setters and getters

100 Vibration API

101 Fluent API

102 Tilde

103 Security issues

104 Get and set CSS custom variables

105 Contributors