How to Manipulate Strings in JavaScript

This tutorial is first published at FreeCodeCamp.

String manipulation is a common task for programmers, whether it is extracting information from the string, converting letter cases, joining strings, or trimming extra white spaces.

This tutorial covers various methods and techniques for manipulating strings using JavaScript, offering you a comprehensive guide on how to work with strings in your JavaScript applications.

How to extract a character from string

Let's start by talking about how to extract a single character from a string. JavaScript offers three different methods for this purpose: charAt(), at(), and charCodeAt().

• charAt(index)

The charAt() method accepts an index, and returns the character at that index.

const str = "Hello World!";
 
console.log(str.charAt(0));
console.log(str.charAt(8));
console.log(str.charAt(16));

H
r
 

If the index is out of range, charAt() will return an empty string ("").

• at(index)

The at() method is added to JavaScript with ES2022, and it is very similar to charAt(). You pass it an index, and the method returns the character at that index.

const str = "Hello World!";
 
console.log(str.at(0));
console.log(str.at(8));
console.log(str.at(16));

H
r
undefined

When the index is out of range, at() will return undefined instead of an empty string.

Another difference is that at() allows for negative indexing, meaning index -1 returns the last character of the string, -2 returns the second last character, and so on.

const str = "Hello World!";
 
console.log(str.at(-1));
console.log(str.at(-2));

!
d

Before at(), the only way to do this is through the length property.

const str = "Hello World!";
 
console.log(str.charAt(str.length - 1)); // The last character
console.log(str.charAt(str.length - 2)); // The second last character

!
d

• charCodeAt(index)

The charCodeAt() method returns the UTF-16 code of the character at the specified index.

const str = "Hello World!";
 
console.log(str.charCodeAt(0));
console.log(str.charCodeAt(4));

72
111

How to extract a substring

Besides extracting a single character, JavaScript also allows you to extract a substring using methods substring() and slice().

• substring(start, end)

substring() extracts a substring based on the provided start (inclusive) and end (exclusive) indexes, and returns the substring as a new string.

const str = "JavaScript";
 
console.log(str.substring(0, 4));

Java

The end index can be left out, in which case the substring will extracted from start to the end of the string.

const str = "JavaScript";
 
console.log(str.substring(4));

Script

• slice(start, end)

slice() is very similar to substring(). It also extracts a substring based on the provided start and end indexes, and returns the substring as a new string.

const str = "JavaScript";
 
console.log(str.slice(0, 4));

Java

The end index can also be omitted.

const str = "JavaScript";
 
console.log(str.slice(4));

Script

The difference is that slice() accepts negative indexes. For example, the following example extracts the substring from index -10 to -6.

const str = "JavaScript";
 
console.log(str.slice(-10, -6));

Java

How to convert string to upper and lower cases

The methods toUpperCase() and toLowerCase() converts the string to upper or lower cases.

const str = "JavaScript";
 
console.log(str.toUpperCase());
console.log(str.toLowerCase());

JAVASCRIPT
javascript

How to join two strings together

The easiest way to join two strings together is using the + operator:

const str1 = "Hello";
const str2 = "World!";
 
const str3 = str1 + " " + str2;
 
console.log(str3);

Hello World!

Alternatively, you can use the concat() method:

const str1 = "Hello";
const str2 = "World!";
 
const str3 = str1.concat(" ", str2);
 
console.log(str3);

Hello World!

Or the template literals:

const str1 = "Hello";
const str2 = "World!";
 
const str3 = `${str1} ${str2}`;
 
console.log(str3);

Hello World!

How to trim extra white spaces from string

When working with strings that came from external sources, such as parsed from a webpage or received from user input, a common problem you might encounter is the leading and trailing white spaces.

JavaScript offers three different methods that allow you to easily remove the extra white spaces and keep only the useful information.

The trimStart() method removes the leading white spaces, including spaces, tabs, and line breaks. The trimEnd() method removes trailing white spaces, and trim() removes white spaces from both ends.

const str = "  \n\tHello World!\t\n  ";
 
console.log(str.trimStart());
console.log(str.trimEnd());
console.log(str.trim());

Hello World!
 
 
  Hello World!
Hello World!

How to add padding to string

The methods padStart() and padEnd() can be used to pad characters or substrings to the beginning or the end of the original string.

Both methods take two arguments, length and a substring. The substring will be repeated multiple times, until the resulting string reaches the target length.

const str = "123";
 
console.log(str.padStart(5, "0"));
console.log(str.padEnd(5, "0"));

00123
12300

If the substring is causing the resulting string to exceed the target length, then only a part of that substring will be used.

const str = "123";
 
console.log(str.padStart(8, "ok"));

okoko123

Notice that the substring "ok" is repeated twice, but for the third time, it causes the resulting string to exceed the length limit, so only "o" is used for the final padding.

How to repeat a string

The repeat() returns a new string, with the specified number of copies of the original string.

const str = "123";
 
console.log(str.repeat(3));

123123123

How to split string into an array

The split() method splits the string based on the given character, and returns the result in an array. This method is most useful when you need to extract information from a URL. For example, this is how you can extract the slug of a blog post:

const url = "http://www.example.com/blog/example-article";
 
let arr = url.split("/");
console.log(arr);
 
let slug = arr[4];
console.log(slug);

[ 'http:', '', 'www.example.com', 'blog', 'example-article' ]
example-article

How to search in a string

You can also search for a character or substring using JavaScript.

• indexOf() and lastIndexOf()

The indexOf() method returns the index of the first occurrence of the given character.

The lastIndexOf() methods returns the index of the last occurrence of the given character.

const str = "Hello World";
 
console.log(str.indexOf("l"));
console.log(str.lastIndexOf("l"));

2
9

Both methods will return -1 if a match is not found.

const str = "Hello World";
 
console.log(str.indexOf("x"));
console.log(str.lastIndexOf("x"));

-1
-1

• includes()

The includes() method tests if the string contains the given character or substring. It returns true if the substring is found, otherwise false will be returned.

const str = "JavaScript";
 
console.log(str.includes("S"));
console.log(str.includes("Script"));
console.log(str.includes("script"));

true
true
false

• startsWith() and endsWith()

As the name suggests, these two methods test if the given substring is found at the beginning or the end of the string.

const str = "JavaScript";
 
console.log(str.startsWith("Java"));
console.log(str.endsWith("Java"));
 
console.log(str.startsWith("Script"));
console.log(str.endsWith("Script"));

true
false
false
true

How to search in a string using Regex

However, what if you need something more powerful? For example, the indexOf() and lastIndexOf() methods only return the first and last occurrences of the substring, but what if you need to search for all of them?

Or what if, instead of a substring, you need to search for a pattern, such as a phone number or a price tag?

This can be achieved by combining the string methods with Regex, which stands for regular expression. It is a programming tool that allows you to describe patterns in a string. Regex has a very cryptic syntax, but can be very useful sometimes.

• search()

The search() method works similarly to indexOf() we just discussed. It also returns the first occurrence of the matched substring or pattern, except that search() allows you to pass a regular expression.

The following example, /(?<=\$)\d\d?\d?\d?/, searches for a price tag in the string, which should start with a dollar sign ($), and followed by 1 to 4 numeric digits.

const str = "The laptop costs $1500. The tablet costs $1000.";
 
console.log(str.search("1500"));
console.log(str.search(/(?<=\$)\d\d?\d?\d?/));
console.log(str.search(/(?<=\$)\d\d?\d?\d?/g));

18
18
18

Notice that the global flag (g) has no effect on search(), and it still returns the first occurrence of the match.

• match() and matchAll()

Compared to search(), the match() method returns much more information that you can work with, such as the actual substring that matches the pattern, the index where the match is found, and more.

const str = "The laptop costs $1500. The tablet costs $1000.";
 
console.log(str.match(/(?<=\$)\d\d?\d?\d?/));
console.log(str.match(/(?<=\$)\d\d?\d?\d?/g));

[
 '1500',
 index: 18,
 input: 'The laptop costs $1500. The tablet costs $1000.',
 groups: undefined
]
[ '1500', '1000' ]

By including a global flag, you can make match() return all matched substrings, instead of just the first one.

There is also a matchAll() method that forces you to use the global flag. Without it, the method will return a TypeError.

matchAll() will return a iterable object, which you can iterate over using a for of loop.

const str = "This laptop costs $1500. The tablet costs $1000.";
 
const prices = str.matchAll(/(?<=\$)\d\d\d\d/g);
 
for (let price of prices) {
  console.log(price);
}

[
 '1500',
 index: 19,
 input: 'This laptop costs $1500. The tablet costs $1000.',
 groups: undefined
]
[
 '1000',
 index: 43,
 input: 'This laptop costs $1500. The tablet costs $1000.',
 groups: undefined
]

How to replace a string pattern

Lastly, the replace() method allows you to match for a pattern, and then replace the matched substrings with a new string. For example,

const str = "JavaScript javaScript Javascript";
 
console.log(str.replace(/JAVASCRIPT/i, "javascript"));
console.log(str.replace(/JAVASCRIPT/gi, "javascript"));

javascript javaScript Javascript
javascript javascript javascript

By default, replace() only matches and replaces the first occurrence of the pattern, but with the global flag, you can replace all matched patterns.

Conclusion

In this tutorial, we explored various methods you can use to work with strings in JavaScript, and also covered how to use regular expressions to match for string patterns.

As a brief summary, here are the methods we discussed in this tutorial:

• charAt(index): Extracts the character at the specified index from a string.
• at(index): Retrieves the character at the specified index, supports negative indexing.
• charCodeAt(index): Returns the UTF-16 code of the character at the specified index.
• substring(start, end): Extracts a part of the string between the start (inclusive) and end indexes (exclusive).
• slice(start, end): Similar to substring(), extracts a part of the string between start (inclusive) and end indexes (exclusive), but supports negative indexing.
• toUpperCase(): Converts all letters in the string to uppercase.
• toLowerCase(): Converts all letters in the string to lowercase.
• concat(): Joins two or more strings together.
• trimStart(): Removes whitespace from the beginning of a string. Including spaces, tabs, and newlines.
• trimEnd(): Removes whitespace from the end of a string.
• trim(): Removes whitespace from both ends of a string.
• padStart(length, substring): Pads the start of a string with another string (multiple times, if needed) until the resulting string reaches the given length.
• padEnd(length, substring): Pads the end of the string with another string (multiple times, if needed) until the resulting string reaches the given length.
• repeat(count): Returns a new string which contains the specified number of copies of the original string.
• split(separator): Splits the string into an array of substrings, using the specified separator to determine where to make the split.
• indexOf(searchValue): Returns the index of the first occurrence of the specified substring. Returns -1 if not found.
• lastIndexOf(searchValue): Returns the index of the last occurrence of the specified substring. Returns -1 if not found.
• includes(searchValue): Determines whether the string contains the specified substring, returning true or false.
• startsWith(searchValue): Checks if the string begins with the specified substring.
• endsWith(searchValue): Checks if the string ends with the specified substring.
• search(regexp): Search for a string pattern, which could be defined by a Regex. Returns the index of the first occurrence of the match or -1 if not found.
• match(regexp): Search for a string pattern, which is defined by a Regex. If a global flag is included, it will return all occurrences of the pattern.
• matchAll(regexp): Returns an iterable object containing all results matching a string against a global regular expression.
• replace(regexp, newSubstr): Replaces occurrences of a pattern (specified by a regular expression) with a new substring.

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