Limit DOM Updates

A common mistake seen in JavaScript when run in a browser environment is updating the DOM more often than necessary.

The issue here is that every update in the DOM interface causes the browser to re-render the screen. If an update changes the layout of an element in the page, the entire page layout needs to be re-computed, and this is very performance-heavy even in the simplest of cases. The process of re-drawing a page is known as reflow and can cause a browser to run slowly or even become unresponsive.

The consequence of updating the document too frequently is illustrated with the following example of adding items to a list.

Consider the following document containing a <ul> element:

<!DOCTYPE html>
<html>
    <body>
        <ul id="list"></ul>
    </body>
</html>

We add 5000 items to the list looping 5000 times (you can try this with a larger number on a powerful computer to increase the effect).

var list = document.getElementById("list");
for (var i = 1; i <= 5000; i++) {             
    list.innerHTML += `<li>item ${i}</li>`;  // update 5000 times
}

In this case, the performance can be improved by batching all 5000 changes in one single DOM update.

var list = document.getElementById("list");
var html = "";
for (var i = 1; i <= 5000; i++) {
    html += `<li>item ${i}</li>`;
}
list.innerHTML = html;     // update once

The function document.createDocumentFragment() can be used as a lightweight container for the HTML created by the loop. This method is slightly faster than modifying the container element’s innerHTML property (as shown below).

var list = document.getElementById("list");
var fragment = document.createDocumentFragment();
for (var i = 1; i <= 5000; i++) {
    li = document.createElement("li");
    li.innerHTML = "item " + i;
    fragment.appendChild(li);
    i++;
}
list.appendChild(fragment);

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

Performance Tips:

* Introduction

* Remarks

* Avoid try/catch in performance-critical functions

* Limit DOM Updates

* Benchmarking your code - measuring execution time

* Cache (memoize) heavy-computing functions

* Initializing object properties with null

* Reuse objects rather than recreate

* Prefer local variables to globals attributes and indexed values

* Be consistent in use of Numbers

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