How to use slice() in JavaScript on an array or a string?

In JavaScript, the slice() function is part of both the Array and the String prototype.

This function is used to select items or characters based on the provided indices.

In this article, you will find all the information needed, for you to become a pro of the slice() function.

I will also provide a lot of common use examples that will help you better understand this function.

Definition

For an array, the slice() built-in function is used to select items based on the provided indices.

For a string, the slice() built-in function is used to select characters based on the provided indices.

By using the start and the end parameters you can specify what portion of the array or string you want to select.

Here is the slice() function in action with an array:

javascriptconst languages = ['english', 'french', 'german', 'spanish', 'russian', 'japanese'];

// This will return: ["spanish", "russian", "japanese"]
console.log(languages.slice(3));

// This will return: ["french", "german"]
console.log(languages.slice(1, 3));

// This will return: ["spanish", "russian", "japanese"]
console.log(languages.slice(-3));

// This will return: ["spanish"]
console.log(languages.slice(3, -2));

Here is the slice() function in action with a string:

javascriptconst str = 'This is a very long string!';

// This will return: "very long string!"
console.log(str.slice(10));

// This will return: "This"
console.log(str.slice(0, 4));

// This will return: "string!"
console.log(str.slice(-7));

// This will return: "string"
console.log(str.slice(-7, -1));

This is the syntax of the slice() function.

arr.slice([ start[, end ]])

Notes

1. The slice() function will NOT modify the original array or string.
2. If you don't specify any parameters, the slice() function will return a copy of the entire array or string.

Parameters

Parameters	Necessity	Description
start	Optional	The start index of the extraction.
end	Optional	The end index before which to extract.

Start Parameter

1. If the start parameter is undefined, the extraction will begin at index ZERO.
2. For an array, if the start parameter is greater than the length of the array, an empty array will be returned.
3. For a string, if the start parameter is greater than the length of the string, an empty string will be returned.

End Parameter

1. For an array, if the end parameter is omitted, the extraction end index will be the length of the array.
2. For a string, if the end parameter is omitted, the extraction end index will be the length of the string.
3. For an array, if the end parameter is greater than the length of the array, the end index will be the length of the array.
4. For a string, if the end parameter is greater than the length of the string, the end index will be the length of the string.

Return Value

On an array, the slice() function will return a new array containing part of the original array.

On a string, the slice() function will return a new string containing part of the original string.

Browser Support

This method works on all browsers since it is an ES1 feature.

Browser	Support
Chrome	YES ✅
Firefox	YES ✅
Opera	YES ✅
Safari	YES ✅
Edge	YES ✅
Internet Explorer	YES ✅

Examples of the slice() function on an array

Here, I've compiled a list of some common use cases for the slice() method on an array.

How to select the first X elements of an array?

To select the first X elements of an array you need to call the slice() function with:

1. A start parameter of ZERO
2. An end parameter

javascriptconst languages = ['english', 'french', 'german', 'spanish', 'russian', 'japanese'];

// This will return: ["english", "french"]
console.log(languages.slice(0, 2));

In this example, we select the first 2 elements from the original array

How to select the last X elements of an array?

To select the last X elements of an array you need to call the slice() function with a positive start parameter.

javascriptconst languages = ['english', 'french', 'german', 'spanish', 'russian', 'japanese'];

// This will return: ["spanish", "russian", "japanese"]
console.log(languages.slice(3));

In this example, we select the last 3 elements.

How to clone an array with the slice() function?

To clone an array you need to call the slice() function without any parameters.

bashconst languages = ['english', 'french', 'german', 'spanish', 'russian', 'japanese'];
const newLanguages = languages.slice();
// This will return: ['english', 'french', 'german', 'spanish', 'russian', 'japanese']
console.log(newLanguages);

In this example, we clone the original array and get a new array.

Examples of the slice() function on a string

I've also compiled a list of some common use cases for the slice() method on a string.

How to get the first X characters of a string?

To get the first X characters of a string you need to call the slice() function with:

1. A start parameter of ZERO
2. An end parameter

javascriptconst str = 'This is a very long string!';

// This will return: "This"
console.log(str.slice(0, 4));

In this example, we get the first 4 characters.

How to get the last X characters of a string?

To get the last X characters of a string you need to call the slice() with a negative start parameter.

javascriptconst str = 'This is a very long string!';

// This will return: "string!"
console.log(str.slice(-7));

In this example, we get the last 7 characters.

slice vs splice

Even tho those two function sounds very alike, they are very different.

The splice() function is used to change the content of an array by:

1. Removing items
2. Replacing existing items/Adding new items in place

The slice() function is used to select items from an array (based on the provided indices).

Here is a table with all the differences:

	splice()	slice()
Return Value	Returns the removed items in an array	Returns the selected items in an array, as a new array
Mutates the original array	YES	NO
Parameters	can take N number of parameters	can take 2 parameters
Parameter 1	Index (required)	The start index of the selection (optional)
Parameter 2	The number of items to be removed (optional)	The end index of the selection (optional)
Parameter 3	The new items to be added (optional)	-

Final thoughts

As you can see the slice() function is an indispensable method when it comes to working with an array or a string in JavaScript.

I am sure you will use it very frequently while developing the next billion-dollar startup.

Thank you for reading!

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