How To Index, Split, and Manipulate Strings in JavaScript
A string is a sequence of one or more characters that may consist of letters, numbers, or symbols. Each character in a JavaScript string can be accessed by an index number, and all strings have methods and properties available to them.
In this tutorial, we will learn the difference between string primitives and the String object, how strings are indexed, how to access characters in a string, and common properties and methods used on strings.
String Primitives and String Objects
First, we will clarify the two types of strings. JavaScript differentiates between the string primitive, an immutable datatype, and the String object.
In order to test the difference between the two, we will initialize a string primitive and a string object.
// Initializing a new string primitive const stringPrimitive = "A new string."; // Initializing a new String object const stringObject = new String("A new string."); We can use the typeof operator to determine the type of a value. In the first example, we simply assigned a string to a variable.
In the second example, we used new String() to create a string object and assign it to a variable.
Most of the time you will be creating string primitives. JavaScript is able to access and utilize the built-in properties and methods of the String object wrapper without actually changing the string primitive you’ve created into an object.
While this concept is a bit challenging at first, you should be aware of the distinction between primitive and object. Essentially, there are methods and properties available to all strings, and in the background JavaScript will perform a conversion to object and back to primitive every time a method or property is called.
How Strings are Indexed
Each of the characters in a string correspond to an index number, starting with 0 .
To demonstrate, we will create a string with the value How are you? .
The first character in the string is H , which corresponds to the index 0 . The last character is ? , which corresponds to 11 . The whitespace characters also have an index, at 3 and 7 .
Being able to access every character in a string gives us a number of ways to work with and manipulate strings.
Accessing Characters
We’re going to demonstrate how to access characters and indices with the How are you? string.
Using square bracket notation, we can access any character in the string.
We can also use the charAt() method to return the character using the index number as a parameter.
Alternatively, we can use indexOf() to return the index number by the first instance of a character.
Although “o” appears twice in the How are you? string, indexOf() will get the first instance.
lastIndexOf() is used to find the last instance.
For both of these methods, you can also search for multiple characters in the string. It will return the index number of the first character in the instance.
The slice() method, on the other hand, returns the characters between two index numbers. The first parameter will be the starting index number, and the second parameter will be the index number where it should end.
Note that 11 is ? , but ? is not part of the returned output. slice() will return what is between, but not including, the last parameter.
If a second parameter is not included, slice() will return everything from the parameter to the end of the string.
To summarize, charAt() and slice() will help return string values based on index numbers, and indexOf() and lastIndexOf() will do the opposite, returning index numbers based on the provided string characters.
Finding the Length of a String
Using the length property, we can return the number of characters in a string.
Remember that the length property is returning the actual number of characters starting with 1, which comes out to 12, not the final index number, which starts at 0 and ends at 11 .
Converting to Upper or Lower Case
The two built-in methods toUpperCase() and toLowerCase() are helpful ways to format text and make textual comparisons in JavaScript.
toUpperCase() will convert all characters to uppercase characters.
toLowerCase() will convert all characters to lowercase characters.
These two formatting methods take no additional parameters.
It is worth noting that these methods do not change the original string.
Splitting Strings
JavaScript has a very useful method for splitting a string by a character and creating a new array out of the sections. We will use the split() method to separate the array by a whitespace character, represented by » » .
const originalString = "How are you?"; // Split string by whitespace character const splitString = originalString.split(" "); console.log(splitString); Now that we have a new array in the splitString variable, we can access each section with an index number.
If an empty parameter is given, split() will create a comma-separated array with each character in the string.
By splitting strings you can determine how many words are in a sentence, and use the method as a way to determine people’s first names and last names, for example.
Trimming Whitespace
The JavaScript trim() method removes white space from both ends of a string, but not anywhere in between. Whitespace can be tabs or spaces.
const tooMuchWhitespace = " How are you? "; const trimmed = tooMuchWhitespace.trim(); console.log(trimmed); The trim() method is a simple way to perform the common task of removing excess whitespace.
Finding and Replacing String Values
We can search a string for a value, and replace it with a new value using the replace() method. The first parameter will be the value to be found, and the second parameter will be the value to replace it with.
const originalString = "How are you?" // Replace the first instance of "How" with "Where" const newString = originalString.replace("How", "Where"); console.log(newString); In addition to being able to replace a value with another string value, we can also use Regular Expressions to make replace() more powerful. For instance, replace() only affects the first value, but we can use the g (global) flag to catch all instances of a value, and the i (case insensitive) flag to ignore case.
const originalString = "Javascript is a programming language. I'm learning javascript." // Search string for "javascript" and replace with "JavaScript" const newString = originalString.replace(/javascript/gi, "JavaScript"); console.log(newString); OutputJavaScript is a programming language. I'm learning JavaScript. This is a very common task that makes use of Regular Expressions. Visit Regexr to practice more examples of RegEx.
Conclusion
Strings are one of the most frequently used data types, and there is a lot we can do with them.
In this tutorial, we learned the difference between the string primitive and String object, how strings are indexed, and how to use the built-in methods and properties of strings to access characters, format text, and find and replace values.
For a more general overview on strings, read the tutorial “How To Work with Strings in JavaScript.”
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Tutorial Series: How To Code in JavaScript
JavaScript is a high-level, object-based, dynamic scripting language popular as a tool for making webpages interactive.
JavaScript Arrays
An array is a special variable, which can hold more than one value:
Why Use Arrays?
If you have a list of items (a list of car names, for example), storing the cars in single variables could look like this:
However, what if you want to loop through the cars and find a specific one? And what if you had not 3 cars, but 300?
An array can hold many values under a single name, and you can access the values by referring to an index number.
Creating an Array
Using an array literal is the easiest way to create a JavaScript Array.
It is a common practice to declare arrays with the const keyword.
Learn more about const with arrays in the chapter: JS Array Const.
Example
Spaces and line breaks are not important. A declaration can span multiple lines:
Example
You can also create an array, and then provide the elements:
Example
Using the JavaScript Keyword new
The following example also creates an Array, and assigns values to it:
Example
The two examples above do exactly the same.
There is no need to use new Array() .
For simplicity, readability and execution speed, use the array literal method.
Accessing Array Elements
You access an array element by referring to the index number:
Note: Array indexes start with 0.
[0] is the first element. [1] is the second element.Changing an Array Element
This statement changes the value of the first element in cars :
Example
Converting an Array to a String
The JavaScript method toString() converts an array to a string of (comma separated) array values.
Example
const fruits = [«Banana», «Orange», «Apple», «Mango»];
document.getElementById(«demo»).innerHTML = fruits.toString();
Access the Full Array
With JavaScript, the full array can be accessed by referring to the array name:
Example
Arrays are Objects
Arrays are a special type of objects. The typeof operator in JavaScript returns «object» for arrays.
But, JavaScript arrays are best described as arrays.
Arrays use numbers to access its «elements». In this example, person[0] returns John:
Array:
Objects use names to access its «members». In this example, person.firstName returns John:
Object:
Array Elements Can Be Objects
JavaScript variables can be objects. Arrays are special kinds of objects.
Because of this, you can have variables of different types in the same Array.
You can have objects in an Array. You can have functions in an Array. You can have arrays in an Array:
Array Properties and Methods
The real strength of JavaScript arrays are the built-in array properties and methods:
Array methods are covered in the next chapters.
The length Property
The length property of an array returns the length of an array (the number of array elements).
Example
The length property is always one more than the highest array index.
Accessing the First Array Element
Example
Accessing the Last Array Element
Example
Looping Array Elements
One way to loop through an array, is using a for loop:
Example
const fruits = [«Banana», «Orange», «Apple», «Mango»];
let fLen = fruits.length;
You can also use the Array.forEach() function:
Example
const fruits = [«Banana», «Orange», «Apple», «Mango»];
Adding Array Elements
The easiest way to add a new element to an array is using the push() method:
Example
const fruits = [«Banana», «Orange», «Apple»];
fruits.push(«Lemon»); // Adds a new element (Lemon) to fruits
New element can also be added to an array using the length property:
Example
const fruits = [«Banana», «Orange», «Apple»];
fruits[fruits.length] = «Lemon»; // Adds «Lemon» to fruits
Adding elements with high indexes can create undefined «holes» in an array:
Example
const fruits = [«Banana», «Orange», «Apple»];
fruits[6] = «Lemon»; // Creates undefined «holes» in fruits
Associative Arrays
Many programming languages support arrays with named indexes.
Arrays with named indexes are called associative arrays (or hashes).
JavaScript does not support arrays with named indexes.
In JavaScript, arrays always use numbered indexes.
Example
const person = [];
person[0] = «John»;
person[1] = «Doe»;
person[2] = 46;
person.length; // Will return 3
person[0]; // Will return «John»
WARNING !!
If you use named indexes, JavaScript will redefine the array to an object.
After that, some array methods and properties will produce incorrect results.
Example:
const person = [];
person[«firstName»] = «John»;
person[«lastName»] = «Doe»;
person[«age»] = 46;
person.length; // Will return 0
person[0]; // Will return undefined
The Difference Between Arrays and Objects
In JavaScript, arrays use numbered indexes.
In JavaScript, objects use named indexes.
Arrays are a special kind of objects, with numbered indexes.
When to Use Arrays. When to use Objects.
- JavaScript does not support associative arrays.
- You should use objects when you want the element names to be strings (text).
- You should use arrays when you want the element names to be numbers.
JavaScript new Array()
JavaScript has a built-in array constructor new Array() .
But you can safely use [] instead.
These two different statements both create a new empty array named points:
These two different statements both create a new array containing 6 numbers:
The new keyword can produce some unexpected results: