【JavaScript Tutorial】JavaScript String

JavaScript Strings

JavaScript strings are used to store and process text.

JavaScript Strings

Strings can store a sequence of characters, such as "John Doe".

Strings can be any characters inserted into quotes. You can use single or double quotes:

instance

var carname = "Volvo XC60";
var carname = 'Volvo XC60';

You can use index position to access each character in a string:

instance

var character = carname[7];

The index of a string starts at 0, which means the first The character index value is [0], the second is [1], and so on.

You can use quotation marks in the string. The quotation marks in the string should not be the same as the quotation marks in the string:

Example

var answer = "It's alright";
var answer = "He is called 'Johnny'";
var answer = 'He is called "Johnny"';

You can also add escape characters to the string to use quotation marks:

Example

var answer = 'It's alright';
var answer = "He is called "Johnny"";

String length

You can use the built-in property length to calculate the length of a string:

Example

var txt = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
var sln = txt.length;

Special characters

In JavaScript, strings are written in single or double quotes.

Because of this, the following example JavaScript cannot be parsed: x

 "We are the so-called "Vikings" from the north."

The string "We are the so-called " is truncated.

How to solve the above problems? You can use backslash () to escape double quotes in the "Vikings" string, as follows:

 "We are the so-called \"Vikings\" from the north."

Backslash is an escape character. Escape characters convert special characters into string characters:

The escape character () can be used to escape apostrophes, newlines, quotes, and other special characters.

The following table lists the special characters that can be escaped using escape characters in strings:

Code

Output

' single quote

" double quote

\ backslash bar

n Line feed

r Carriage return

t tab (tab)

b Backspace character

f Form feed character

String can be an object

Normally, JavaScript strings are primitive values , can be created using characters: var firstName = "John"

But we can also use the new keyword to define the string as an object: var firstName = new String("John")

Instance

var x = "John";
var y = new String("John");
typeof x // returns String
typeof y // returns Object

Do not create a String object . It slows down execution and may have other side effects:

Instance

var x = "John";              
var y = new String("John");
(x === y) // is false because x is a string and y is an object.

String properties and methods

Primitive value strings, like "John", have no properties and methods

Primitive values ​​can use JavaScript properties and methods, because JavaScript can treat primitive values ​​as objects when executing methods and properties.

String methods we will introduce in the next chapter

Properties.

Description

constructor Returns a function that creates a string attribute property

length Returns the length of a string

prototype Allows you to add properties and methods to an object

String method

Method

Description

charAt() Returns the character at the specified index position

charCodeAt() Returns the Unicode value of the character at the specified index position

concat() Concatenates two or more characters String, return the concatenated string

fromCharCode() Convert characters to Unicode values ​​

indexOf() Return the position where the specified character appears for the first time in the string

lastIndexOf() Return the last index of the specified character in the string An occurrence of a position

localeCompare() Compares two strings in locale-specific order

match() Finds a match for one or more regular expressions

replace() Replaces a substring that matches a regular expression

search() Retrieve values ​​that match a regular expression

slice() Extract a fragment of a string and return the extracted part in a new string

split() Split the string into an array of substrings

substr() Extract the specified number of characters from the string from the starting index number

substring() Extract the characters between the two specified index numbers in the string

toLocaleLowerCase() Convert the string according to the locale of the host Convert to lowercase, only a few languages ​​(such as Turkish) have local-specific case mapping

toLocaleUpperCase() Convert the string to uppercase according to the locale of the host, only a few languages ​​(such as Turkish) have local-specific case mapping Case mapping

toLowerCase() Convert the string to lowercase

toString() Return the string object value

toUpperCase() Convert the string to uppercase

trim() Remove the leading and trailing blanks of the string

valueOf( ) Return the original value of a string object

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