JavaScript String Type
Summary: in this tutorial, you will learn about the JavaScript String type and how to manipulate strings effectively.
Introduction to JavaScript String type
The String type is object wrapper of the string primitive type and can be created by using the String constructor as follows:
let str = new String('JavaScript String Type');Code language: JavaScript (javascript)
The String type has a property named length that specifies the number of characters in the string.
console.log(str.length); // 22Code language: JavaScript (javascript)
In this example, the value of the length property is 22 that also is the number of characters in the string 'JavaScript String Type'.
To get the primitive string value, you use one of the following methods of the String object: valueOf(), toString(), and toLocaleString().
console.log(str.valueOf());
console.log(str.toString());
console.log(str.toLocaleString());Code language: CSS (css)
To access an individual character in a string, you use square bracket notation [] with a numeric index. The index of the first character is zero as shown in this example:
console.log(str[0]); // JCode language: JavaScript (javascript)
The square bracket notation was introduced in ES5. Prior to ES5, you use the charAt() method, which is more verbose:
console.log(str.charAt(0)); // JCode language: JavaScript (javascript)
When you call a method on a primitive string variable or a literal string, it is converted to an instance of the String type. For example:
'literal string'.toUpperCase();Code language: JavaScript (javascript)
This feature is known as primitive wrapper types in JavaScript.
String manipulation
The String type provides many useful methods for manipulating strings effectively. We will examine each of them in the following section.
1) Concatenating strings
To concatenate two or more strings you use the concat() method as follows:
let firstName = 'John';
let fullName = firstName.concat(' ','Doe');
console.log(fullName); // "John Doe"
console.log(firstName); // "John"Code language: JavaScript (javascript)
The concat() method concatenates two or more strings and returns the result string. Note that the concat() method does not modify the original string.
Besides the concat() method, you can also use the addition operator (+) for concatenating strings. In practice, the addition operator is used more often than the concat() method.
let firstName = 'John';
let fullName = firstName + ' ' + 'Doe';
console.log(fullName); // "John Doe"Code language: JavaScript (javascript)
2) Extracting substrings
To extract a substring from a string, you use the substr() method:
substr(startIndex,[length]);Code language: CSS (css)
The substr() method accepts two arguments.
The first argument startIndex is the location at which the characters are being extracted, while the second argument length specifies the number of characters to extract.
let str = "JavaScript String";console.log(str.substr(0, 10)); // "JavaScript"
console.log(str.substr(11,6)); // "String"
Code language: JavaScript (javascript)
If you omit the length argument, the substr() method extracts the characters to the end of the string.
Sometimes, you want to extract a substring from a string using starting and ending indexes. In this case, you use the substring() method:
substring(startIndex,endIndex)
See the following example:
let str = "JavaScript String";
console.log(str.substring(4, 10)); // "Script"Code language: JavaScript (javascript)
3) Locating substrings
To locate a substring in a string, you use the indexOf() method:
string.indexOf(substring,[fromIndex]);Code language: CSS (css)
The indexOf() method accepts two arguments: a substring to locate and the fromIndex at which the method starts searching forward in the string.
The indexOf() returns the index of the first occurrence of the substring in the string. If the substring is not found, the indexOf() method returns -1.
let str = "This is a string";
console.log(str.indexOf("is")); // 2Code language: JavaScript (javascript)
The following example uses the fromIndex argument:
console.log(str.indexOf('is', 3)); //5Code language: JavaScript (javascript)
To find the location of the last occurrence of a substring in a string, you use the lastIndexOf() method.
string.lastIndexOf(substring,[fromIndex])Code language: CSS (css)
Unlike the indexOf() method, the lastindexOf() method searches backward from the fromIndex argument.
console.log(str.lastIndexOf('is')); // 5Code language: JavaScript (javascript)
The lastIndexOf() method also returns -1 if the substring is not found in the string as shown in this example:
console.log(str.lastIndexOf('are')); // -1Code language: JavaScript (javascript)
4) Removing whitespace characters
To remove all leading and trailing whitespace characters of a string, you use the trim() method.
let rawString = ' Hi ';
let strippedString = rawString.trim();
console.log(strippedString); // "Hi"Code language: JavaScript (javascript)
Note that the trim() method creates a copy of the original string with whitespace characters stripped, it doesn’t change the original string.
ES6 introduced two new methods for removing whitespace characters from a string:
trimStart()returns a string with whitespace stripped from the beginning of a string.trimEnd()returns a string with the whitespace stripped from the end of a string.
5) Changing cases
To change cases of a string, you use toLowerCase() and toUpperCase() methods:
let greeting = 'Hello';
console.log(greeting.toLowerCase()); // 'hello'
console.log(greeting.toUpperCase()); // 'HELLO';Code language: JavaScript (javascript)
In some languages, the rules for converting a string to lowercase and uppercase are very specific.
Therefore, it is safer to use the toLocaleLowerCase() and toLocaleUpperCase() methods, especially when you don’t know which language the code will deal with.
6) Comparing strings
To compare two strings, you use the localeCompare() method:
first.localeCompare(second);Code language: CSS (css)
The localeCompare() returns one of three values: -1, 0, and 1.
- If the first string comes before the second string alphabetically, the method returns -1.
- If the first string comes after the second string alphabetically, the method returns 1.
- If two strings are equal, the method returns 0.
For example:
console.log('A'.localeCompare('B')); // -1
console.log('B'.localeCompare('B')); // 0
console.log('C'.localeCompare('B')); // 1Code language: JavaScript (javascript)
7) Matching patterns
The match() method allows you to match a string with a specified regular expression and get an array of results.
The match() method returns null if it does not find any match. Otherwise, it returns an array containing the entire match and any parentheses-capture matched results.
If the global flag (g) is not set, the element zero of the array contains the entire match. Here is an example:
let expression = '1 + 2 = 3';
let matches = expression.match(/\d+/);
console.log(matches[0]); // "1"Code language: JavaScript (javascript)
Output:
1
In this example, the pattern matches any number in the expression string.
In case the global flag ( g) is set, the elements of the result array contain all matches as follows:
let expression = '1 + 2 = 3';
let matches = expression.match(/\d+/g);for (const match of matches) {
console.log(match);
}
Code language: JavaScript (javascript)
Output:
1
2
3
In this example, the matches array contains all the matches including 1, 2, and 3 in the expression string.
If you only need to find out if a string matches a regular expression, you use the search() method instead.
Similar to the match() method, the search() method accepts a regular expression and returns the position of the string where the first match is found. In case no match is found, it returns -1.
let str = "This is a test of search()";
let pos = str.search(/is/);
console.log(pos); // 2Code language: JavaScript (javascript)
8) Replacing substrings
To replace a substring in a string, you use the replace() method.
string.replace(regularExpression, replaceText)Code language: CSS (css)
The first argument of the replace() method could be a regular expression or a string. If it is a regular expression, the replace() method will find the matches and replace them with the second argument ( replaceText). In case it is a string, the replace() method will perform an exact search and carry the replacement.
The replace() method returns a copy of the original string after making the replacements.
The following example illustrates how to use a regular expression to replace the with a:
let str = "the baby kicks the ball";// replace "the" with "a"
let newStr = str.replace(/the/g, "a");
console.log(newStr); // "a baby kicks a ball"
console.log(str); // "the baby kicks the ball"
Code language: JavaScript (javascript)
And the following example shows how to replace kicks with holds by using the first argument as a literal string.
newStr = str.replace('kicks', 'holds');
console.log(newStr); // "the baby holds the ball"Code language: JavaScript (javascript)
In this tutorial, you have learned about the JavaScript String type and how to manipulate strings effectively.