Essential JavaScript String Methods

The 13 most important JavaScript functions for working with strings. Index, slice, split, and trim your way through JS strings methods.

Strings are a fundamental part of every programming language, and JavaScript has many powerful built-in functions that make working with strings easy for developers. This list covers the most important string functions for you to begin utilizing in your code.

.length — Finding the length of the string

This one may seem obvious, but it is likely the most important string method and undoubtedly the most commonly used one. Calling .length on a string will return the number of character that the string contains.

.trim() — Removing white space

The trim() function removes white space from the beginning and end of a string. You will find yourself using this one most often when processing the string of a user input field. It is easy to accidentally add spaces, and this ensures you handle the relevant characters.

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.includes() — Determine if string contains substring

The includes() function determines if a substring is contained in a larger string and returns true or false . This has many applications, but one common use-case is for string matching for searching/parsing.

.indexOf() — Finding the index of a substring

Before includes() was introduced to the JavaScript spec, indexOf() was the primary way you would check if a substring existed in a string. It is likely you will still see code that uses indexOf , so understanding how it works is important. The function returns the index of the substring within the string. If the substring is not contained in the original string, it will return -1 .

The common pattern for indexOf() which mimics the behavior of includes() which is to check if the index is greater than -1:

.toUpperCase() — Capitalizes entire string

The toUpperCase() function returns a string with all upper case letters.

.toLowerCase() — Lower cases entire string

The toLowerCase() function returns a string with all lower case letters.

.replace() — Replaces strings with new values

The replace() function is called on a string and will return a string with a pattern replaced by a replacement string. It takes either a regex or a string as the pattern . With a regex you can globally replace all matches (using the g option), but with a string it will only replace the first occurrence. In the example below, you will notice that world is replaced only once in the first call since it uses a string pattern.

.slice() — Return a section of a string

The slice() method will extract a section of a string based on the index supplied and return it as a new string. This is useful when you know the structure of a string and want to retrieve a certain portion, or it can be used with the indexOf method which we learned earlier where you can find the index of the first occurrence of a substring and use that as a reference point for slicing.

slice() takes the beginning index as the first parameter and an optional ending index as the second parameter — str.slice(beginIndex[, endIndex]) . If no ending index is supplied, it slices to the end of the string starting with your beginIndex . If a negative beginIndex is used, it will slice backwards from the end of the string. The following is an example from MDN which depicts these cases.

.split() — Converts string into an array of strings

The split() method takes a separator which you want to split apart the string on, and it returns an array of strings. This is useful when you know your string uses a certain character to separate data, or if you want to operate on specific substrings individually.

.repeat() — Repeats a string a specified number of times

The repeat() function returns a string consisting of the elements of the object repeated the given number of times.

.match() — Returns array of matching strings

The match() method retrieves the matches when matching a string against a regular expression . The example below searches our string for all capital letters. It returns an array of strings for the values that match the regex.

.charAt() — Returns the character at an index

The charAt() function returns the string character at a given index.

.charCodeAt() —Return the unicode at an index

The charCodeAt() method returns the unicode of the character at a specified index in a string. This an UTF-16 cone integer between 0 and 65535.

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