Detecting a string

To detect whether a parameter is a primitive string, use typeof:

var aString = "my string";
var anInt = 5;
var anObj = {};
typeof aString === "string";   // true
typeof anInt === "string";     // false
typeof anObj === "string";     // false

If you ever have a String object, via new String("somestr"), then the above will not work. In this instance, we can use instanceof:

var aStringObj = new String("my string");
aStringObj instanceof String;    // true

To cover both instances, we can write a simple helper function:

var isString = function(value) {
    return typeof value === "string" || value instanceof String;
};

var aString = "Primitive String";
var aStringObj = new String("String Object");
isString(aString); // true
isString(aStringObj); // true
isString({}); // false
isString(5); // false

Or we can make use of toString function of Object. This can be useful if we have to check for other types as well say in a switch statement, as this method supports other datatypes as well just like typeof.

var pString = "Primitive String";
var oString = new String("Object Form of String");
Object.prototype.toString.call(pString);//"[object String]"
Object.prototype.toString.call(oString);//"[object String]"

A more robust solution is to not detect a string at all, rather only check for what functionality is required. For example:

var aString = "Primitive String";
// Generic check for a substring method
if (aString.substring) {

}
// Explicit check for the String substring prototype method
if (aString.substring === String.prototype.substring) {
    aString.substring(0, );
}

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

Strings:

* Syntax

* Basic info and string concatenation

* Reverse a string

* Access character at index in string

* Comparing strings lexicographically

* Word counter

* Escaping quotes

* Splitting a string into an array

* Strings are unicode

* Detecting a string

* Trim whitespace

* Character code

* String representations of numbers

* Substring with slice

* Find and replace

* Find the index of a substring inside a string

* String to lower case

* Repeat a string

* String to upper case

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